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diff --git a/old/3772-h/files/ch17.html b/old/3772-h/files/ch17.html new file mode 100644 index 0000000..c37022f --- /dev/null +++ b/old/3772-h/files/ch17.html @@ -0,0 +1,1303 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> +<!-- saved from url=(0036)http://../Lyell/The Student's Elements of Geology --> +<html> +<head> +<meta name="generator" content="HTML Tidy, see www.w3.org"> +<title>The Student's Elements of Geology: Title</title> +<meta content="text/html; charset=iso-8859-1" http-equiv= +"Content-Type"> +<meta content="MSHTML 5.00.2919.6307" name="GENERATOR"> +<link rel="stylesheet" href="geology.css" type="text/css"> +</head> +<body> +<p><b>The Student’s Elements of Geology</b></p> + +<hr> +<p class="page"><a name="page 281">[ 281 ]</a></p> + +<p> </p> + +<center> +<h3>SECONDARY OR MESOZOIC SERIES</h3> + +<hr width="40%"> +<br> +<br> +<b>Chapter XVII</b><br> +<br> +UPPER CRETACEOUS GROUP.</center> + +<p class="intro">Lapse of Time between Cretaceous and Eocene +Periods. — Table of successive Cretaceous Formations. — +Maestricht Beds. — Pisolitic Limestone of France. — +Chalk of Faxoe. — Geographical Extent and Origin of the White +Chalk. — Chalky Matter now forming in the Bed of the +Atlantic. — Marked Difference between the Cretaceous and +existing Fauna. — Chalk-flints. — Pot-stones of +Horstead. — Vitreous Sponges in the Chalk. — Isolated +Blocks of Foreign Rocks in the White Chalk supposed to be +ice-borne. — Distinctness of Mineral Character in +contemporaneous Rocks of the Cretaceous Epoch. — Fossils of +the White Chalk. — Lower White Chalk without Flints. — +Chalk Marl and its Fossils. — Chloritic Series or Upper +Greensand. — Coprolite Bed near Cambridge. — Fossils of +the Chloritic Series. — Gault. — Connection between +Upper and Lower Cretaceous Strata. — Blackdown Beds. — +Flora of the Upper Cretaceous Period. — Hippurite Limestone. +— Cretaceous Rocks in the United States.</p> + +<p>We have treated in the preceding chapters of the Tertiary or +Cainozoic strata, and have next to speak of the Secondary or +Mesozoic formations. The uppermost of these last is commonly called +the chalk or the cretaceous formation, from creta, the latin name +for that remarkable white earthy limestone, which constitutes an +upper member of the group in those parts of Europe where it was +first studied. The marked discordance in the fossils of the +tertiary, as compared with the cretaceous formations, has long +induced many geologists to suspect that an indefinite series of +ages elapsed between the respective periods of their origin. +Measured, indeed, by such a standard, that is to say, by the amount +of change in the Fauna and Flora of the earth effected in the +interval, the time between the Cretaceous and Eocene may have been +as great as that between the Eocene and Recent periods, to the +history of which the last seven chapters have been devoted. Several +deposits have been met with here and there, in the course of the +last half century, of an age intermediate between the white chalk +and the plastic clays and sands of the Paris and London districts, +monuments</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 282">[ 282 ]</a></p> + +<p>which have the same kind of interest to a geologist which +certain medieval records excite when we study the history of +nations. For both of them throw light on ages of darkness, preceded +and followed by others of which the annals are comparatively +well-known to us. But these newly-discovered records do not fill up +the wide gap, some of them being closely allied to the Eocene, and +others to the Cretaceous type, while none appear as yet to possess +so distinct and characteristic a fauna as may entitle them to hold +an independent place in the great chronological series.</p> + +<p>Among the formations alluded to, the Thanet Sands of Prestwich +have been sufficiently described in the last chapter, and classed +as Lower Eocene. To the same tertiary series belong the Belgian +formations, called by Professor Dumont, Landenian. On the other +hand, the Maestricht and Faxoe limestones are very closely +connected with the chalk, to which also the Pisolitic limestone of +France is referable.</p> + +<p><b>Classification of the Cretaceous Rocks.</b>—The +cretaceous group has generally been divided into an Upper and a +Lower series, the Upper called familiarly <i>the chalk,</i> and the +Lower <i>the greensand</i>; the one deriving its name from the +predominance of white earthy limestone and marl, of which it +consists in a great part of France and England, the other or lower +series from the plentiful mixture of green or chloritic grains +contained in some of the sands and cherts of which it largely +consists in the same countries. But these mineral characters often +fail, even when we attempt to follow out the same continuous +subdivisions throughout a small portion of the north of Europe, and +are worse than valueless when we desire to apply them to more +distant regions. It is only by aid of the organic remains which +characterise the successive marine subdivisions of the formation +that we are able to recognise in remote countries, such as the +south of Europe or North America, the formations which were there +contemporaneously in progress. To the English student of geology it +will be sufficient to begin by enumerating those groups which +characterise the series in this country and others immediately +contiguous, alluding but slightly to those of more distant regions. +In the table (p. 283) it will be seen that I have used the term +Neocomian for that commonly called “Lower Greensand;” +as this latter term is peculiarly objectionable, since the green +grains are an exception to the rule in many of the members of this +group even in districts where it was first studied and named.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 283">[ 283 ]</a></p> + +<center> +<table border="1" cellpadding="2" width="80%"> +<tr> +<td colspan="2" align="center">UPPER CRETACEOUS OR CHALK +PERIOD.</td> +</tr> + +<tr> +<td colspan="2"> +<ol> +<li>Maestricht Beds and Faxoe Limestone.</li> + +<li>Upper White Chalk, with flints.</li> + +<li>Lower White Chalk, without flints.</li> + +<li>Chalk Marl.</li> + +<li>Chloritic series (or Upper Greensand).</li> + +<li>Gault.</li> +</ol> +</td> +</tr> + +<tr> +<td colspan="2" align="center">LOWER CRETACEOUS OR NEOCOMIAN.</td> +</tr> + +<tr> +<td align="center">Marine</td> +<td align="center">Fresh-water</td> +</tr> + +<tr> +<td align="left"> +<ol> +<li>Marine: Upper Neocomian, see <a href="ch18.html#page 308"> +p.308</a></li> + +<li>Marine: Middle Neocomian, see <a href="ch18.html#page 312"> +p.312</a></li> + +<li>Marine: Lower Neocomian, see <a href="ch18.html#page 312"> +p.312</a></li> +</ol> +</td> +<td valign="middle" align="left">Wealden Beds (upper part).</td> +</tr> +</table> +</center> + +<br> + <img src="../images2/fig226.jpg" width="144" height="279" alt= +"Belemnitella mucronata." align="right"> + +<p><i>Maestricht Beds.</i>—On the banks of the Meuse, at +Maestricht, reposing on ordinary white chalk with flints, we find +an upper calcareous formation about 100 feet thick, the fossils of +which are, on the whole, very peculiar, and all distinct from +tertiary species. Some few are of species common to the inferior +white chalk, among which may be mentioned <i>Belemnitella +mucronata</i> (Fig. 226) and <i>Pecten quadricostatus,</i> a shell +regarded by many as a mere variety of <i>P. quinquecostatus</i> +(see <a href="../images2/fig270.jpg">Fig. 270</a>). Besides the +Belemnite there are other <i>genera,</i> such as <i>Baculites</i> +and <i>Hamites,</i> never found in strata newer than the +cretaceous, but frequently met with in these Maestricht beds. On +the other hand, <i>Voluta, Fasciolaria,</i> and other genera of +univalve shells, usually met with only in tertiary strata, +occur.</p> + +<p>The upper part of the rock, about 20 feet thick, as seen in St. +Peter’s Mount, in the suburbs of Maestricht, abounds in +corals and Bryozoa, often detachable from the matrix; and these +beds are succeeded by a soft yellowish limestone 50 feet thick, +extensively quarried from time immemorial for building. The stone +below is whiter, and contains occasional nodules of grey chert or +chalcedony.</p> + +<p>M. Bosquet, with whom I examined this formation (August, 1850), +pointed out to me a layer of chalk from two to four inches thick, +containing green earth and numerous encrinital stems, which forms +the line of demarkation between the strata containing the fossils +peculiar to Maestricht and</p> + +<p class="fnote">* For particulars of structure see <a href= +"ch18.html#page 318">p. 318.</a></p> + +<p> </p> + +<hr> +<p class="page"><a name="page 284">[ 284 ]</a></p> + +<center><img src="../images2/fig227.jpg" width="379" height="254" alt= +"Mosasaurus Camperi."></center> + +<p>the white chalk below. The latter is distinguished by regular +layers of black flint in nodules, and by several shells, such as +<i>Terebratula carnea</i> (see <a href="../images2/fig240.jpg">Fig. +246</a>), wholly wanting in beds higher than the green band. Some +of the organic remains, however, for which St. Peter’s Mount +is celebrated, occur both above and below that parting layer, and, +among others, the great marine reptile called <i>Mosasaurus</i> +(see Fig. 227), a saurian supposed to have been 24 feet in length, +of which the entire skull and a great part of the skeleton have +been found. Such remains are chiefly met with in the soft +freestone, the principal member of the Maestricht beds. Among the +fossils common to the Maestricht and white chalk may be instanced +the echinoderm, Fig. 228.</p> + +<img src="../images2/fig228.jpg" width="164" height="180" alt= +"Hemipneustes radiatus." align="left"> + +<p>I saw proofs of the previous denudation of the white chalk +exhibited in the lower bed of the Maestricht formation in Belgium, +about 30 miles S.W. of Maestricht, at the village of Jendrain, +where the base of the newer deposit consisted chiefly of a layer of +well-rolled, black chalk-flint pebbles, in the midst of which +perfect specimens of <i>Thecidea papillata</i> and <i>Belemnitella +mucronata</i> are imbedded. To a geologist accustomed in England to +regard rolled pebbles of chalk-flint as a common and distinctive +feature of tertiary beds of different ages, it is a new and +surprising phenomenon to behold strata made up of such materials, +and yet to feel no doubt that they were</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 285">[ 285 ]</a></p> + +<p>accumulated in a sea in which the belemnite and other cretaceous +mollusca flourished.</p> + +<p><b>Pisolitic Limestone of France.</b>—Geologists were for +many years at variance respecting the chronological relations of +this rock, which is met with in the neighbourhood of Paris, and at +places north, south, east, and west of that metropolis, as between +Vertus and Laversines, Meudon and Montereau. By many able +palæontologists the species of fossils, more than fifty in +number, were declared to be more Eocene in their appearance than +Cretaceous. But M. Hébert found in this formation at +Montereau, near Paris, the <i>Pecten quadricostatus,</i> a +well-known Cretaceous species, together with some other fossils +common to the Maestricht chalk and to the Baculite limestone of the +Cotentin, in Normandy. He therefore, as well as M. Alcide +d’Orbigny, who had carefully studied the fossils, came to the +opinion that it was an upper member of the Cretaceous group. It is +usually in the form of a coarse yellowish or whitish limestone, and +the total thickness of the series of beds already known is about +100 feet. Its geographical range, according to M. Hébert, is +not less than 45 leagues from east to west, and 35 from north to +south. Within these limits it occurs in small patches only, resting +unconformably on the white chalk.</p> + +<p>The <i>Nautilus Danicus,</i> <a href="../images2/fig229.jpg">Fig. +230,</a> and two or three other species found in this rock, are +frequent in that of Faxoe, in Denmark, but as yet no Ammonites, +Hamites, Scaphites, Turrilites, Baculites, or Hippurites have been +met with. The proportion of peculiar species, many of them of +tertiary aspect, is confessedly large; and great aqueous erosion +suffered by the white chalk, before the pisolitic limestone was +formed, affords an additional indication of the two deposits being +widely separated in time. The pisolitic formation, therefore, may +eventually prove to be somewhat more intermediate in date between +the secondary and tertiary epochs than the Maestricht rock.</p> + +<p><b>Chalk of Faxoe.</b>— In the island of Seeland, in +Denmark, the newest member of the chalk series, seen in the +sea-cliffs at Stevensklint resting on white chalk with flints, is a +yellow limestone, a portion of which, at Faxoe, where it is used as +a building stone, is composed of corals, even more conspicuously +than is usually observed in recent coral reefs. It has been +quarried to the depth of more than 40 feet, but its thickness is +unknown. The imbedded shells are chiefly casts, many of them of +univalve mollusca, which are usually very rare in the white chalk +of Europe. Thus, there are two species of <i>Cypræa,</i> one +of <i>Oliva,</i> two of <i>Mitra,</i> four of the genus</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 286">[ 286 ]</a></p> + +<p><i>Cerithium,</i> six of <i>Fusus,</i> two of <i>Trochus,</i> +one of <i>Patella,</i> one of <i>Emarginula,</i> etc.; on the +whole, more than thirty univalves, spiral or patelliform. At the +same time, some of the accompanying bivalve shells, echinoderms, +and zoophytes, are specifically identical with fossils of the true +Cretaceous series. Among the cephalopoda of Faxoe may be mentioned +<i>Baculites Faujasii</i> (Fig. 229), and <i>Belemnitella +mucronata</i> (<a href="../images2/fig226.jpg">Fig. 226</a>), shells +of the white chalk. The <i>Nautilus Danicus</i> (see Fig. 230) is +characteristic of this formation; and it also occurs in France in +the calcaire pisolitique of Laversin (Department of Oise). The +claws and entire skull of a small crab, <i>Brachyurus rugosus</i> +(Schlott.), are scattered through the Faxoe stone, reminding us of +similar crustaceans inclosed in the rocks of modern coral reefs. +Some small portions of this coralline formation consist of white +earthy chalk.</p> + +<center><img src="../images2/fig229.jpg" width="420" height="208" alt= +"Fig. 229: Portion of Baculites Faujasii, Fig. 230: Nautilus Danicus."> +</center> + +<p><b>Composition, Extent and Origin of the White +Chalk.</b>—The highest beds of chalk in England and France +consist of a pure, white, calcareous mass, usually too soft for a +building-stone, but sometimes passing into a more solid state. It +consists, almost purely, of carbonate of lime; the stratification +is often obscure, except where rendered distinct by interstratified +layers of flint, a few inches thick, occasionally in continuous +beds, but oftener in nodules, and recurring at intervals generally +from two to four feet distant from each other. This upper chalk is +usually succeeded, in the descending order, by a great mass of +white chalk without flints, below which comes the chalk marl, in +which there is a slight admixture of argillaceous matter. The +united thickness of the three divisions in the south of England +equals, in some places, 1000 feet. The section in <a href= +"../images2/fig231.jpg">Fig. 231</a> will show the manner in which the +white chalk extends from England into France, covered by the +tertiary strata described in former chapters, and reposing on lower +cretaceous beds.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 287">[ 287 ]</a></p> + +<p>The area over which the white chalk preserves a nearly +homogeneous aspect is so vast, that the earlier geologists +despaired of discovering any analogous deposits of recent date. +Pure chalk, of nearly uniform aspect and composition, is met with +in a north-west and south-east direction, from the north of Ireland +to the Crimea, a distance of about 1140 geographical miles, and in +an opposite direction it extends from the south of Sweden to the +south of Bordeaux, a distance of about 840 geographical miles. In +Southern Russia, according to Sir R. Murchison, it is sometimes 600 +feet thick, and retains the same mineral character as in France and +England, with the same fossils, including <i>Inoceramus Cuvieri, +Belemnitella mucronata,</i> and <i>Ostrea vesicularis</i> (<a href= +"../images2/fig251.jpg">Fig. 251).</a></p> + +<center><img src="../images2/fig231.jpg" width="609" height="100" alt= +"Diagrammatic section from Hertfordshire, in England, to Sens, in France."> +</center> + +<p>Great light has recently been thrown upon the origin of the +unconsolidated white chalk by the deep soundings made in the North +Atlantic, previous to laying down, in 1858, the electric telegraph +between Ireland and Newfoundland. At depths sometimes exceeding two +miles, the mud forming the floor of the ocean was found, by +Professor Huxley, to be almost entirely composed (more than +nineteen-twentieths of the whole) of minute Rhizopods, or +foraminiferous shells of the genus Globigerina, especially the +species <i>Globigerina bulloides</i> (see <a href= +"../images2/fig232.jpg">Fig. 232.</a>) the organic bodies next in +quantity were the siliceous shells called <i> +Polycystineæ,</i> and next to them the siliceous skeletons of +plants called <i>Diatomaceæ</i> (<a href= +"../images2/fig232.jpg">Figs. 233, 234, 235</a>), and occasionally +some siliceous spiculæ of sponges (<a href= +"../images2/fig232.jpg">Fig. 236</a>) were intermixed. These were +connected by a mass of living gelatinous matter to which he has +given the name of <i>Bathybius,</i> and which contains abundance of +very minute bodies termed Coccoliths and Coccospheres, which have +also been detected fossil in chalk.</p> + +<p>Sir Leopold MacClintock and Dr. Wallich have ascertained that 95 +per cent of the mud of a large part of the North Atlantic consists +of Globigerina shells. But Captain Bullock, <small>R.N.</small>, +lately brought up from the enormous depth of 16,860 feet a +white,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 288">[ 288 ]</a></p> + +<center><img src="../images2/fig232.jpg" width="348" height="185" alt= +"Fig. 232: Globigerina bulloides, Calcareous Rhizopod. Fig. 233: Actinocyclus, Fig. 234: Pinnularia, Fig. 235: Eunotia bidens, Siliceous Diatomaceæ. Fig. 236: Spicula of sponge, Siliceous sponge."> +</center> + +<p>viscid, chalky mud, wholly devoid of Globigerinæ. This mud +was perfectly homogeneous in composition, and contained no organic +remains visible to the naked eye. Mr. Etheridge, however, has +ascertained by microscopical examination that it is made up of <i> +Coccoliths, Discoliths,</i> and other minute fossils like those of +the Chalk classed by Huxley as <i>Bathybius,</i> when this term is +used in its widest sense. This mud, more than three miles deep, was +dredged up in latitude 20° 19' N., longitude 4° 36' E., or +about midway between Madeira and the Cape of Good Hope.</p> + +<p>The recent deep-sea dredgings in the Atlantic conducted by Dr. +Wyville Thomson, Dr. Carpenter, Mr. Gwyn Jeffreys, and others, have +shown that on the same white mud there sometimes flourish Mollusca, +Crustacea, and Echinoderms, besides abundance of siliceous sponges, +forming, on the whole, a marine fauna bearing a striking +resemblance in its general character to that of the ancient +chalk.</p> + +<p><b>Popular Error as to the Geological Continuity of the +Cretaceous Period.</b>—We must be careful, however, not to +overrate the points of resemblance which the deep-sea +investigations have placed in a strong light. They have been +supposed by some naturalists to warrant a conclusion expressed in +these words: “We are still living in the Cretaceous +epoch;” a doctrine which has led to much popular delusion as +to the bearing of the new facts on geological reasoning and +classification. The reader should be reminded that in geology we +have been in the habit of founding our great chronological +divisions, not on foraminifera and sponges, nor even on echinoderms +and corals, but on the remains of the most highly organised beings +available to us, such as the mollusca; these being met with, as +explained (<a href="ch9.html#page 142">p. 142</a>), in stratified +rocks of almost every age. In dealing with the mollusca, it is +those of the highest or most specialised organisation, which afford +us the best characters in proportion as their vertical range is the +most limited. Thus the Cephalopoda</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 289">[ 289 ]</a></p> + +<p>are the most valuable, as having a more restricted range in time +than the Gasteropoda; and these, again, are more characteristic of +the particular stratigraphical subdivisions than are the +Lamellibranchiate Bivalves, while these last, again, are more +serviceable in classification than the Brachiopoda, a still lower +class of shell-fish, which are the most enduring of all.</p> + +<p>When told that the new dredgings prove that “we are still +living in the Chalk Period,” we naturally ask whether some +cuttle-fish has been found with a Belemnite forming part of its +internal framework; or have Ammonites, Baculites, Hamites, +Turrilites, with four or five other Cephalopodous genera +characteristic of the chalk and unknown as tertiary, been met with +in the abysses of the ocean? Or, in the absence of these +long-extinct forms, has a single spiral univalve, or species of +Cretaceous Gasteropod, been found living? Or, to descend still +lower in the scale, has some characteristic Cretaceous genus of +Lamellibranchiate Bivalve, such as the Inoceramus, or Hippurite, +foreign to the Tertiary seas, been proved to have survived down to +our time? Or, of the numerous genera of lamellibranchiates common +to the Cretaceous and Recent seas, has one species been found +living? The answer to all these questions is—not one has been +found. Even of the humblest shell-fish, the Brachiopods, no new +species common to the Cretaceous and recent seas has yet been met +with. It has been very generally admitted by conchologists that out +of a hundred species of this tribe occurring fossil in the Upper +Chalk—one, and one only, <i>Terebratulina striata,</i> is still +living, being thought to be identical with <i>Terebratula +caput-serpentis.</i> Although this identity is still questioned by +some naturalists of authority, it would certainly not surprise us +if another lamp-shell of equal antiquity should be met with in the +deep sea.</p> + +<p>Had it been declared that we are living in the Eocene epoch, the +idea would not be so extravagant, for the great reptiles of the +Upper Chalk, the Mosasaurus, Pliosaurus, and Pterodactyle, and many +others, as well as so many genera of chambered univalves, had +already disappeared from the earth, and the marine fauna had made a +greater approach to our own by nearly the entire difference which +separates it from the fauna of the Cretaceous seas. The Eocene +nummulitic limestone of Egypt is a rock mainly composed, like the +more ancient white chalk, of globigerine mud; and if the reader +will refer to what we have said of the extent to which the +nummulitic marine strata, formed originally at the bottom of the +sea, now enter into the frame-work of</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 290">[ 290 ]</a></p> + +<p>mountain chains of the principal continents, he will at once +perceive that the present Atlantic, Pacific, and Indian Oceans are +geographical terms, which must be wholly without meaning when +applied to the Eocene, and still more to the Cretaceous Period; so +that to talk of the chalk having been uninterruptedly forming in +the Atlantic from the Cretaceous Period to our own, is as +inadmissible in a geographical as in a geological sense.</p> + +<p><b>Chalk-flints.</b>—The origin of the layers of flint, +whether in the form of nodules, or continuous sheets, or in veins +or cracks not parallel to the stratification, has always been more +difficult to explain than that of the white chalk. But here, again, +the late deep-sea soundings have suggested a possible source of +such mineral matter. During the cruise of the +“Bulldog,” already alluded to, it was ascertained that +while the calcareous <i>Globigerinæ</i> had almost exclusive +possession of certain tracts of the sea-bottom, they were wholly +wanting in others, as between Greenland and Labrador. According to +Dr. Wallich, they may flourish in those spaces where they derive +nutriment from organic and other matter, brought from the south by +the warm waters of the Gulf Stream, and they may be absent where +the effects of that great current are not felt. Now, in several of +the spaces where the calcareous Rhizopods are wanting, certain +microscopic plants, called <i>Diatomaceæ,</i> above mentioned +(<a href="../images2/fig232.jpg">Figs. 233-235</a>), the solid parts +of which are siliceous, monopolise the ground at a depth of nearly +400 fathoms, or 2400 feet.</p> + +<p>The large quantities of silex in solution required for the +formation of these plants may probably arise from the +disintegration of feldspathic rocks, which are universally +distributed. As more than half of their bulk is formed of siliceous +earth, they may afford an endless supply of silica to all the great +rivers which flow into the ocean. We may imagine that, after a +lapse of many years or centuries, changes took place in the +direction of the marine currents, favouring at one time a supply in +the same area of siliceous, and at another of calcareous matter in +excess, giving rise in the one case to a preponderance of +Globigerinæ, and in the other of Diatomaceæ. These +last, and certain sponges, may by their decomposition have +furnished the silex, which, separating from the chalky mud, +collected round organic bodies, or formed nodules, or filled +shrinkage cracks.</p> + +<p><b>Pot-stones.</b>—A more difficult enigma is presented by +the occurrence of certain huge flints, or pot-stones, as they are +called in Norfolk, occurring singly, or arranged in nearly +continuous columns at right angles to the ordinary and</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 291">[ 291 ]</a></p> + +<p>horizontal layers of small flints. I visited in the year 1825 an +extensive range of quarries then open on the river Bure, near +Horstead, about six miles from Norwich, which afforded a continuous +section, a quarter of a mile in length, of white chalk, exposed to +the depth of about twenty-six feet, and covered by a bed of gravel. +The pot-stones, many of them pear-shaped, were usually about three +feet in height and one foot in their transverse diameter, placed in +vertical rows, like pillars, at irregular distances from each +other, but usually from twenty to thirty feet apart, though +sometimes nearer together, as in Figure 237. These rows did not +terminate downward in any instance which I could examine, nor +upward, except at the point where they were cut off abruptly by the +bed of gravel. On breaking open the pot-stones, I found an internal +cylindrical nucleus of pure chalk, much harder than the ordinary +surrounding chalk, and not crumbling to pieces like it, when +exposed to the winter’s frost. At the distance of half a +mile, the vertical piles of pot-stones were much farther apart from +each other. Dr. Buckland has described very similar phenomena as +characterising the white chalk on the north coast of Antrim, in +Ireland.*</p> + +<center><img src="../images2/fig237.jpg" width="390" height="307" alt= +"View of a chalk-pit at Horstead, near Norwich, showing the position of the pot-stones."> +</center> + +<p><b>Vitreous Sponges of the Chalk.</b>—These pear-shaped +masses of flint often resemble in shape and size the large +sponges</p> + +<p class="fnote">* Geol. Trans., 1st Series, vol. iv, p. 413.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 292">[ 292 ]</a></p> + +<p>called Neptune’s Cups (<i>Spongia patera,</i> Hardw.), +which grow in the seas of Sumatra; and if we could suppose a series +of such gigantic sponges to be separated from each other, like +trees in a forest, and the individuals of each successive +generation to grow on the exact spot where the parent sponge died +and was enveloped in calcareous mud, so that they should become +piled one above the other in a vertical column, their growth +keeping pace with the accumulation of the enveloping calcareous +mud, a counterpart of the phenomena of the Horstead pot-stones +might be obtained.</p> + +<img src="../images2/fig238.jpg" width="123" height="281" alt= +"Fig. 238: Ventriculites radiatus. White chalk." align="left"> + +<p>Professor Wyville Thomson, describing the modern soundings in +1869 off the north coast of Scotland, speaks of the ooze or chalk +mud brought from a depth of about 3000 feet, and states that at one +haul they obtained forty specimens of vitreous sponges buried in +the mud. He suggests that the Ventriculites of the chalk were +nearly allied to these sponges, and that when the silica of their +spicules was removed, and was dissolved out of the calcareous +matrix, it set into flint.</p> + +<p><b>Boulders and Groups of Pebbles in Chalk.</b>—The +occurrence here and there, in the white chalk of the south of +England, of isolated pebbles of quartz and green schist has justly +excited much wonder. It was at first supposed that they had been +dropped from the roots of some floating tree, by which means stones +are carried to some of the small coral islands of the Pacific. But +the discovery in 1857 of a group of stones in the white chalk near +Croydon, the largest of which was syenite and weighed about forty +pounds, accompanied by pebbles and fine sand like that of a beach, +has been shown by Mr. Godwin Austen to be inexplicable except by +the agency of floating ice. If we consider that icebergs now reach +40 degrees north latitude in the Atlantic, and several degrees +nearer the equator in the southern hemisphere, we can the more +easily believe that even during the Cretaceous epoch, assuming that +the climate was milder, fragments of coast ice may have floated +occasionally as far as the south of England.</p> + +<p><b>Distinctness of Mineral Character in Contemporaneous Rocks of +the Cretaceous Period.</b>—But we must not imagine that +because pebbles are so rare in the white chalk of England and +France there are no proofs of sand, shingle, and clay having</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 293">[ 293 ]</a></p> + +<p>been accumulated contemporaneously even in European seas. The +siliceous sandstone called “upper quader” by the +Germans overlies white argillaceous chalk or +“pläner-kalk,” a deposit resembling in composition +and organic remains the chalk marl of the English series. This +sandstone contains as many fossil shells common to our white chalk +as could be expected in a sea-bottom formed of such different +materials. It sometimes attains a thickness of 600 feet, and, by +its jointed structure and vertical precipices, plays a conspicuous +part in the picturesque scenery of Saxon Switzerland, near Dresden. +It demonstrates that in the Cretaceous sea, as in our own, distinct +mineral deposits were simultaneously in progress. The quartzose +sandstone alluded to, derived from the detritus of the neighbouring +granite, is absolutely devoid of carbonate of lime, yet it was +formed at the distance only of four hundred miles from a sea-bottom +now constituting part of France, where the purely calcareous white +chalk was forming. In the North American continent, on the other +hand, where the Upper Cretaceous formations are so widely +developed, true white chalk, in the ordinary sense of that term, +does not exist.</p> + +<center><img src="../images2/fig239.jpg" width="321" height="189" alt= +"Fig. 239: Ananchytes ovatus. White chalk, upper and lower."> +</center> + +<p><b>Fossils of the White Chalk.</b>—Among the fossils of +the white chalk, echinoderms are very numerous; and some of the +genera, like <i>Ananchytes</i> (see Fig. 239), are exclusively +cretaceous. Among the Crinoidea, the <i>Marsupites</i> (<a href= +"../images2/fig240.jpg">Fig. 242</a>) is a characteristic genus. Among +the mollusca, the cephalopoda are represented by Ammonites, +Baculites (<a href="../images2/fig229.jpg">Fig. 229</a>), and +Belemnites (<a href="../images2/fig226.jpg">Fig. 226</a>). Although +there are eight or more species of Ammonites and six of them +peculiar to it, this genus is much less fully represented than in +each of the other subdivisions of the Upper Cretaceous group.</p> + +<p>Among the brachiopoda in the white chalk, the <i> +Terebratulæ</i> are very abundant (see <a href= +"../images2/fig240.jpg">Figs. 243-247</a>). With these</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 294">[ 294 ]</a></p> + +<p>are associated some forms of oyster (see <a href= +"../images2/fig251.jpg">Fig. 251</a>), and other bivalves (Figs. 249, +250).</p> + +<center><img src="../images2/fig240.jpg" width="432" height="519" alt= +"Fig. 240: Micraster cor-angumum. White chalk. Fig. 241: Galerites albogalerus. White chalk. Fig. 242: Marsupites Milleri. White chalk. Fig. 243: Terebratulina striata. Upper white chalk. Fig. 244: Rhynchonella octoplicata. Upper white chalk. Fig. 245: Magas pumila. Upper white chalk. Fig. 246: Terebratula carnea. Upper white chalk. Fig. 247: Terebratula biplicata. Upper cretaceous. Fig. 248: Crania Parisiensis. Inferior or attached valve. Upper white chalk. Fig. 249: Peten Beaveri. Lower white chalk and chalk marl. Fig. 250: Lima spinosa. Upper white chalk."> +</center> + +<p>Among the bivalve mollusca, no form marks the Cretaceous era in +Europe, America, and India in a more striking manner than the +extinct genus <i>Inoceramus</i> (<i>Catillus</i> of Lam.; see <a +href="../images2/fig251.jpg">Fig. 252</a>), the shells of which are +distinguished by a fibrous texture, and are often met with in +fragments, having probably been extremely friable.</p> + +<p>Of the singular family called <i>Rudistes</i> by Lamarck, +hereafter to be mentioned as extremely characteristic of the +chalk</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 295">[ 295 ]</a></p> + +<p>of southern Europe, a single representative only (Fig. 253) has +been discovered in the white chalk of England.</p> + +<center><img src="../images2/fig251.jpg" width="368" height="300" alt= +"Fig. 251: Ostrea vesicularis. Upper chalk and upper greensand. Fig. 252: Inoceramus Lamarckii. White chalk. Figs. 253, 254, 255, 256: Radiolites Mortoni. White chalk."> +</center> + +<br> +<br> +The general absence of univalve mollusca in the white chalk is very +marked. Of bryozoa there is an abundance, such as <i>Eschara</i> +and <i>Escharina</i> (<a href="../images2/fig257.jpg">Figs. 257, +258</a>). These and other organic bodies, especially sponges, such +as <i>Ventriculites</i><br> +<br> + + +<p> </p> + +<hr> +<p class="page"><a name="page 296">[ 296 ]</a></p> + +<p>(<a href="../images2/fig238.jpg">Fig. 238</a>), are dispersed +indifferently through the soft chalk and hard flint, and some of +the flinty nodules owe their irregular forms to inclosed sponges, +such as Fig. 259, <i>a,</i> where the hollows in the exterior are +caused by the branches of a sponge (Fig. 259, <i>b</i>), seen on +breaking open the flint.</p> + +<center><img src="../images2/fig257.jpg" width="441" height="368" alt= +"Fig. 257: Eschara disticha. White chalk. Fig. 258: Escharina oceani. White chalk. Fig. 259: A branching sponge in a flint, from the white chalk."> +</center> + +<p>The remains of fishes of the Upper Cretaceous formations consist +chiefly of teeth belonging to the shark family. Some of the genera +are common to the Tertiary formations, and some are distinct. To +the latter belongs the genus <i>Ptychodus</i> (<a href= +"../images2/fig260.jpg">Fig. 260</a>), which is allied to the living +Port Jackson shark, <i>Cestracion Phillippi,</i> the anterior teeth +of which (see <a href="../images2/fig261.jpg">Fig. 261,</a> <i>a</i>) +are sharp and cutting, while the posterior or palatal teeth +(<i>b</i>) are flat (<a href="../images2/fig260.jpg">Fig. 260</a>). +But we meet with no bones of land-animals, nor any terrestrial or +fluviatile shells, nor any plants, except sea-weeds, and here and +there a piece of drift-wood. All the appearances concur</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 297">[ 297 ]</a></p> + +<p>in leading us to conclude that the white chalk was the product +of an open sea of considerable depth.</p> + +<img src="../images2/fig260.jpg" width="124" height="147" alt= +"Fig. 260: Palatal tooth of Ptychodus decurrens. Lower white chalk." + align="left"> + +<p>The existence of turtles and oviparous saurians, and of a +Pterodactyl or winged lizard, found in the white chalk of +Maidstone, implies, no doubt, some neighbouring land; but a few +small islets in mid-ocean, like Ascension, formerly so much +frequented by migratory droves of turtle, might perhaps have +afforded the required retreat where these creatures laid their eggs +in the sand, or from which the flying species may have been blown +out to sea. Of the vegetation of such islands we have scarcely any +indication, but it consisted partly of cycadaceous plants; for a +fragment of one of these was found by Captain Ibbetson in the Chalk +Marl of the Isle of Wight, and is referred by A. Brongniart to <i> +Clathraria Lyellii,</i> Mantell, a species common to the antecedent +Wealden period. The fossil plants, however, of beds corresponding +in age to the white chalk at Aix-la-Chapelle, presently to be +described, like the sandy beds of Saxony, before alluded to (<a +href="#page 293">p. 293</a>), afford such evidence of land as to +prove how vague must be any efforts of ours to restore the +geography of that period.</p> + +<img src="../images2/fig261.jpg" width="275" height="267" alt= +"Fig. 261: Cestracion Phillipi; recent." align="right"> + +<p>The Pterodactyl of the Kentish chalk, above alluded to, was of +gigantic dimensions, measuring 16 feet 6 inches from tip to tip of +its outstretched wings. Some of its elongated bones were at first +mistaken by able anatomists for those of birds; of which class no +osseous remains have as yet been derived from the white chalk, +although they have been found (as will be seen on page 299) in the +Chloritic sand.</p> + +<p>The collector of fossils from the white chalk was formerly +puzzled by meeting with certain bodies which they call larch-cones, +which were afterwards recognised by Dr. Buckland</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 298">[ 298 ]</a></p> + +<p>to be the excrement of fish (see Fig. 262). They are composed in +great part of phosphate of lime.</p> + +<center><img src="../images2/fig262.jpg" width="398" height="258" alt= +"Fig. 262: Coprolites of fish, from the chalk. Fig. 263: Baculites anceps. Lower chalk. Fig. 264: Ammonites Rhotomagensis. Chalk marl."> +</center> + +<p><b>Lower White Chalk.</b>—The Lower White Chalk, which is +several hundred feet thick, without flints, has yielded 25 species +of Ammonites, of which half are peculiar to it. The genera +Baculite, Hamite, Scaphite, Turrilite, Nautilus, Belemnite, and +Belemnitella, are also represented.</p> + +<p><b>Chalk Marl.</b>—The lower chalk without flints passes +gradually downward, in the south of England, into an argillaceous +limestone, “the chalk marl,” already alluded to. It +contains 32 species of Ammonites, seven of which are peculiar to +it, while eleven pass up into the overlying lower white chalk. <i> +A. Rhotomagensis</i> is characteristic of this formation. Among the +British cephalopods of other genera may be mentioned <i>Scaphites +æqualis</i> (<a href="../images2/fig265.jpg">Fig. 266</a>) and +<i>Turrilites costatus</i> (<a href="../images2/fig265.jpg">Fig. +265</a>).</p> + +<p><b>Chloritic Series (or Upper Greensand).</b>—According to +the old nomenclature, this subdivision of the chalk was called +Upper Greensand, in order to distinguish it from those members of +the Neocomian or Lower Cretaceous series below the Gault to which +the name of Greensand had been applied. Besides the reasons before +given (<a href="#page 282">p. 282</a>) for abandoning this +nomenclature, it is objectionable in this instance as leading the +uninitiated to suppose that the divisions thus named Upper and +Lower Greensand are of co-ordinate value, instead of which the +chloritic sand is quite a subordinate member of the Upper +Cretaceous group, and the term Greensand has very commonly been +used for the whole of the Lower Cretaceous rocks, which are almost +comparable in importance to</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 299">[ 299 ]</a></p> + +<img src="../images2/fig265.jpg" width="278" height="299" alt= +"Fig. 265: Turrilites costatus. Lower chalk and chalk marl. Fig. 266: Scaphites æqualis. Chloritic marl and sand, Dorsetshire." + align="right"> + +<p>the entire Upper Cretaceous series. The higher portion of the +Chloritic series in some districts has been called chloritic marl, +from its consisting of a chalky marl with chloritic grains. In +parts of Surrey, where calcareous matter is largely intermixed with +sand, it forms a stone called malm-rock or firestone. In the cliffs +of the southern coast of the Isle of Wight it contains bands of +calcareous limestone with nodules of chert.</p> + +<p><i>Coprolite Bed.</i>—The so-called coprolite bed, found +near Farnham, in Surrey, and near Cambridge, contains nodules of +phosphate of lime in such abundance as to be largely worked for the +manufacture of artificial manure. It belongs to the upper part of +the Chloritic series, and is doubtless chiefly of animal origin, +and may perhaps be partly coprolitic, derived from the excrement of +fish and reptiles. The late Mr. Barrett discovered in it, near +Cambridge, in 1858, the remains of a bird, which was rather larger +than the common pigeon, and probably of the order Natatores, and +which, like most of the Gull tribe, had well-developed wings. +Portions of the metacarpus, metatarsus, tibia, and femur have been +detected, and the determinations of Mr. Barrett have been confirmed +by Professor Owen.</p> + +<p>This phosphatic bed in the suburbs of Cambridge must have been +formed partly by the denudation of pre-existing rocks, mostly of +Cretaceous age. The fossil shells and bones of animals washed out +of these denuded strata, now forming a layer only a few feet thick, +have yielded a rich harvest to the collector. A large Rudist of the +genus Radiolite, no less than two feet in height, may be seen in +the Cambridge Museum, obtained from this bed. The number of +reptilian remains, all apparently of Cretaceous age, is truly +surprising; more than ten species of Pterodactyl, five or six of +Ichthyosaurus, one of Pliosaurus, one of Dinosaurus, eight of +Chelonians, besides other forms, having been recognised.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 300">[ 300 ]</a></p> + +<p>The chloritic sand is regarded by many geologists as a littoral +deposit of the Chalk Ocean, and therefore contemporaneous with part +of the chalk marl, and even, perhaps, with some part of the white +chalk. For, as the land went on sinking, and the cretaceous sea +widened its area, white mud and chloritic sand were always forming +somewhere, but the line of sea-shore was perpetually shifting its +position. Hence, though both sand and mud originated +simultaneously, the one near the land, the other far from it, the +sands in every locality where a shore became submerged might +constitute the underlying deposit.</p> + +<center><img src="../images2/fig267.jpg" width="388" height="169" alt= +"Fig. 267: Ostrea columba. Chloritic sand. Fig. 268: Ostrea carinata. Chalk marl and chloritic sand."> +</center> + +<p>Among the characteristic mollusca of the chloritic sand may be +mentioned <i>Terebrirostra lyra</i> (Fig. 269), <i>Plagiostoma +Hoperi</i> (Fig. 271), <i>Pecten quinque-costatus</i> (Fig. 270), +and <i>Ostrea columba</i> (Fig. 267).</p> + +<center><img src="../images2/fig269.jpg" width="375" height="194" alt= +"Fig. 269: Terebrirostra lyra. Chloritic sand. Fig. 270: Pecten 5-costatus. White chalk and chloritic sand. Fig. 271: Plagiostoma Hoperi. White chalk and chloritic sand."> +</center> + +<p>The Cephalopoda are abundant, among which 40 species of +Ammonites are now known, 10 being peculiar to this subdivision, and +the rest common to the beds immediately above or below.</p> + +<p><b>Gault.</b>—The lowest member of the Upper Cretaceous +group,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 301">[ 301 ]</a></p> + +<img src="../images2/fig272.jpg" width="242" height="196" alt= +"Fig. 272: Ancyloceras spinigerum. Near Folkestone." align="right"> + + +<p>usually about 100 feet thick in the S.E. of England, is +provincially termed Gault. It consists of a dark blue marl, +sometimes intermixed with green sand. Many peculiar forms of +cephalopoda, such as the <i>Hamite</i> (Fig. 272), and <i> +Scaphite,</i> with other fossils, characterise this formation, +which, small as is its thickness, can be traced by its organic +remains to distant parts of Europe, as, for example, to the +Alps.</p> + +<p>Twenty-one species of British Ammonites are recorded as found in +the Gault, of which only eight are peculiar to it, ten being common +to the overlying Chloritic series.</p> + +<p><b>Connection between Upper and Lower Cretaceous +Strata.—Blackdown Beds.</b>—The break between the Upper +and Lower Cretaceous formations will be appreciated when it is +stated that, although the Neocomian contains 31 species of +Ammonite, and the Gault, as we have seen, 21, there are only three +of those common to both divisions. Nevertheless, we may expect the +discovery in England, and still more when we extend our survey to +the Continent, of beds of passage intermediate between the Upper +and Lower Cretaceous. Even now the Blackdown beds in Devonshire, +which rest immediately on Triassic strata, and which evidently +belong to some part of the Cretaceous series, have been referred by +some geologists to the Upper group, by others to the Lower or +Neocomian. They resemble the Folkestone beds of the latter series +in mineral character, and 59 out of 156 of their fossil mollusca +are common to them; but they have also 16 species common to the +Gault, and 20 to the overlying Chloritic series; and what is very +important, out of seven Ammonites six are found also in the Gault +and Chloritic series, only one being peculiar to the Blackdown +beds.</p> + +<p>Professor Ramsay has remarked that there is a stratigraphical +break; for in Kent, Surrey, and Sussex, at those few points where +there are exposures of junctions of the Gault and Neocomian, the +surface of the latter has been much eroded or denuded, while to the +westward of the great chalk escarpment the unconformability of the +two groups is equally striking. At Blackdown this unconformability +is still more marked, for though distant only 100 miles from</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 302">[ 302 ]</a></p> + +<p>Kent and Surrey, no formation intervenes between these beds and +the Trias; all intermediate groups, such as the Lower Neocomian and +Oolite, having either not been deposited or destroyed by +denudation.</p> + +<p><b>Flora of the Upper Cretaceous Period.</b>—As the Upper +Cretaceous rocks of Europe are, for the most part, of purely marine +origin, and formed in deep water usually far from the nearest +shore, land-plants of this period, as we might naturally have +anticipated, are very rarely met with. In the neighbourhood of +Aix-la-Chapelle, however, an important exception occurs, for there +certain white sands and laminated clays, 400 feet in thickness, +contain the remains of terrestrial plants in a beautiful state of +preservation. These beds are the equivalents of the white chalk and +chalk marl of England, or Senonien of d’Orbigny, although the +white siliceous sands of the lower beds, and the green grains in +the upper part of the formation, cause it to differ in mineral +character from our white chalk.</p> + +<p>Beds of fine clay, with fossil plants, and with seams of +lignite, and even perfect coal, are intercalated. Floating wood, +containing perforating shells, such as Pholas and Gastrochoena, +occur. There are likewise a few beds of a yellowish-brown +limestone, with marine shells, which enable us to prove that the +lowest and highest plant-beds belong to one group. Among these +shells are <i>Pecten quadricostatus,</i> and several others which +are common to the upper and lower part of the series, and <i> +Trigonia limbata,</i> D’Orbigny, a shell of the white chalk. +On the whole, the organic remains and the geological position of +the strata prove distinctly that in the neighbourhood of +Aix-la-Chapelle a gulf of the ancient Cretaceous sea was bounded by +land composed of Devonian rocks. These rocks consisted of quartzose +and schistose beds, the first of which supplied white sand and the +other argillaceous mud to a river which entered the sea at this +point, carrying down in its turbid waters much drift-wood and the +leaves of plants. Occasionally, when the force of the river abated, +marine shells of the genera <i>Trigonia, Turritella, Pecten,</i> +etc., established themselves in the same area, and plants allied to +<i>Zostera</i> and <i>Fucus</i> grew on the bottom.</p> + +<p>The fossil plants of this member of the upper chalk at Aix have +been diligently collected and studied by Dr. Debey, and as they +afford the only example yet known of a terrestrial flora older than +the Eocene, in which the great divisions of the vegetable kingdom +are represented in nearly the same proportions as in our own times, +they deserve particular attention. Dr. Debey estimates the number +of species</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 303">[ 303 ]</a></p> + +<p>as amounting to more than two hundred, of which sixty-seven are +cryptogamous, chiefly ferns, twenty species of which can be well +determined, most of them being in fructification. The scars on the +bark of one or two are supposed to indicate tree-ferns. Of thirteen +genera three are still existing, namely, <i>Gleichenia,</i> now +inhabiting the Cape of Good Hope, and New Holland; Lygodium, now +spread extensively through tropical regions, but having some +species which live in Japan and North America; and <i> +Asplenium,</i> a cosmopolite form. Among the phænogamous +plants, the Conifers are abundant, the most common belonging to a +genus called Cycadopteris by Debey, and hardly separable from +Sequoia (or Wellingtonia), of which both the cones and branches are +preserved. When I visited Aix, I found the silicified wood of this +plant very plentifully dispersed through the white sands in the +pits near that city. In one silicified trunk 200 rings of annual +growth could be counted. Species of Araucaria like those of +Australia are also found. Cycads are extremely rare, and of +Monocotyledons there are but few. No palms have been recognised +with certainty, but the genus Pandanus, or screw pine, has been +distinctly made out. The number of the Dicotyledonous Angiosperms +is the most striking feature in so ancient a flora.*</p> + +<p>Among them we find the familiar forms of the Oak, Fig, and +Walnut (Quercus, Ficus, and Juglans), of the last both the nuts and +leaves; also several genera of the Myrtaceæ. But the +predominant order is the Proteaceæ, of which there are +between sixty and seventy supposed species, many of extinct genera, +but some referred to the following living forms—Dryandra, +Grevillea, Hakea, Banksia, Persoonia—all</p> + +<p class="fnote">* In this and subsequent remarks on fossil plants +I shall often use Dr. Lindley’s terms, as most familiar in +this country; but as those of M. A. Brongniart are much cited, it +may be useful to geologists to give a table explaining the +corresponding names of groups so much spoken of in +palæontology.</p> + +<table class="note" border="1" cellpadding="4" cellspacing="0" +summary="Table explaining the corresponding names of groups so much spoken of in palæontology."> +<tr> +<td> </td> +<td align="center">Brongniart.</td> +<td align="center">Lindley.</td> +<td> </td> +</tr> + +<tr> +<td valign="middle" align="left" rowspan="2">Cryptogamic.</td> +<td align="left">1. Cryptogamous amphigens, or cellular +cryptogamic.</td> +<td align="left" valign="top">Thallogens.</td> +<td align="left" valign="top">Lichens, sea-weeds, fungi.</td> +</tr> + +<tr> +<td align="left">2. Cryptogamous acrogens.</td> +<td align="left">Acrogens.</td> +<td align="left">Mosses, equisetums, ferns, +lycopodiums,—Lepidodendra.</td> +</tr> + +<tr> +<td align="left" valign="middle" rowspan="3"> +Phænerogamic.</td> +<td align="left">3. Dicotyledonous gymnosperms.</td> +<td align="left" valign="top">Gymnogens.</td> +<td align="left" valign="top">Conifers and Cycads.</td> +</tr> + +<tr> +<td align="left" valign="top">4. Dicot. angiosperms.</td> +<td align="left" valign="top">Exogens.</td> +<td align="left" valign="top">Compositæ, leguminosæ, +cruciferæ, healths, etc. All native European trees except +conifers.</td> +</tr> + +<tr> +<td align="left" valign="top">5. Monocotyledons.</td> +<td align="left" valign="top">Endogens.</td> +<td align="left" valign="top">Palms, lilies, aloes, rushes, +grasses, etc.</td> +</tr> +</table> + +<p> </p> + +<hr> +<p class="page"><a name="page 304">[ 304 ]</a></p> + +<p>now belonging to Australia, and Leucospermum, species of which +form small bushes at the Cape.</p> + +<p>The epidermis of the leaves of many of these Aix plants, +especially of the Proteaceæ, is so perfectly preserved in an +envelope of fine clay, that under the microscope the stomata, or +polygonal cellules, can be detected, and their peculiar arrangement +is identical with that known to characterise some living +Proteaceæ (Grevillea, for example). Although this peculiarity +of the structure of stomata is also found in plants of widely +distant orders, it is, on the whole, but rarely met with, and being +thus observed to characterise a foliage previously suspected to be +proteaceous, it adds to the probability that the botanical evidence +had been correctly interpreted.</p> + +<p>An occasional admixture at Aix-la-Chapelle of Fucoids and +Zosterites attests, like the shells, the presence of salt-water. Of +insects, Dr. Debey has obtained about ten species of the families +Curculionidæ and Carabidæ.</p> + +<p>The resemblance of the flora of Aix-la-Chapelle to the tertiary +and living floras in the proportional number of dicotyledonous +angiosperms as compared to the gymnogens, is a subject of no small +theoretical interest, because we can now affirm that these Aix +plants flourished before the rich reptilian fauna of the secondary +rocks had ceased to exist. The Ichthyosaurus, Pterodactyl, and +Mosasaurus were of coeval date with the oak, the walnut, and the +fig. Speculations have often been hazarded respecting a connection +between the rarity of Exogens in the older rocks and a peculiar +state of the atmosphere. A denser air, it was suggested, had in +earlier times been alike adverse to the well-being of the higher +order of flowering plants, and of the quick-breathing animals, such +as mammalia and birds, while it was favourable to a cryptogamic and +gymnospermous flora, and to a predominance of reptile life. But we +now learn that there is no incompatibility in the co-existence of a +vegetation like that of the present globe, and some of the most +remarkable forms of the extinct reptiles of the age of +gymnosperms.</p> + +<p>If the passage seem at present to be somewhat sudden from the +flora of the Lower or Neocomian to that of the Upper Cretaceous +period, the abruptness of the change will probably disappear when +we are better acquainted with the fossil vegetation of the +uppermost beds of the Neocomian and that of the lowest strata of +the Gault or true Cretaceous series.</p> + +<p><b>Hippurite limestone.</b>—<i>Difference between the +Chalk of the North and South of Europe.</i>—By the aid of the +three tests,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 305">[ 305 ]</a></p> + +<img src="../images2/fig273.jpg" width="178" height="319" alt= +"Fig. 273: Map." align="right"> + +<p>superposition, mineral character, and fossils, the geologist has +been enabled to refer to the same Cretaceous period certain rocks +in the north and south of Europe, which differ greatly both in +their fossil contents and in their mineral composition and +structure.</p> + +<p>If we attempt to trace the cretaceous deposits from England and +France to the countries bordering the Mediterranean, we perceive, +in the first place, that in the neighbourhood of London and Paris +they form one great continuous mass, the Straits of Dover being a +trifling interruption, a mere valley with chalk cliffs on both +sides. We then observe that the main body of the chalk which +surrounds Paris stretches from Tours to near Poitiers (see Fig. +273, in which the shaded part represents chalk).</p> + +<p>Between Poitiers and La Rochelle, the space marked A on the map +separates two regions of chalk. This space is occupied by the +Oolite and certain other formations older than the Chalk and +Neocomian, and has been supposed by M. E. de Beaumont to have +formed an island in the Cretaceous sea. South of this space we +again meet with rocks which we at once recognise to be cretaceous, +partly from the chalky matrix and partly from the fossils being +very similar to those of the white chalk of the north: especially +certain species of the genera <i>Spatangus, Ananchytes, Cidarites, +Nucula, Ostrea,</i></p> + +<center><img src="../images2/fig274.jpg" width="373" height="177" alt= +"Fig. 274: Radiolites. White chalk of France. Fig. 275: Radiolites foliaceus. White chalk of France."> +</center> + +<p> </p> + +<hr> +<p class="page"><a name="page 306">[ 306 ]</a></p> + +<p><i>Gryphæa (Exogyra), Pecten, Plagiostoma (Lima), +Trigonia, Catillus (Inoceramus),</i> and <i>Terebratula.</i>* But +Ammonites, as M. d’Archiac observes, of which so many species +are met with in the chalk of the north of France, are scarcely ever +found in the southern region; while the genera <i>Hamite, +Turrilite,</i> and <i>Scaphite,</i> and perhaps <i>Belemnite,</i> +are entirely wanting.</p> + +<img src="../images2/fig276.jpg" width="254" height="443" alt= +"Fig. 276: Hippurites organisans. Upper chalk:—chalk marl of Pyrenees?†" + align="left"> + +<p>On the other hand, certain forms are common in the south which +are rare or wholly unknown in the north of France. Among these may +be mentioned many <i>Hippurites, Sphærulites,</i> and other +members of that great family of mollusca called <i>Rudistes</i> by +Lamarck, to which nothing analogous has been discovered in the +living creation, but which is quite characteristic of rocks of the +Cretaceous era in the south of France, Spain, Sicily, Greece, and +other countries bordering the Mediterranean. The species called <i> +Hippurites organisans</i> (Fig. 276) is more abundant than any +other in the south of Europe; and the geologist should make himself +well acquainted with the cast of the interior, <i>d,</i> which is +often the only part preserved in many compact marbles of the Upper +Cretaceous period. The flutings on the interior of the Hippurite, +which are represented on the cast by smooth, rounded longitudinal +ribs, and in some individuals attain a great size and length, are +wholly unlike the markings on the exterior of the shell.</p> + +<p class="fnote">* D’Archiac, Sur la form. +Crétacée du S.-O. de la France Mém. de la Soc. +Géol. de France, tome ii.<br> +† D’Orbigny’s Paléontologie +français, pl. 533.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 307">[ 307 ]</a></p> + +<p><b>Cretaceous Rocks in the United States.</b>—If we pass +to the American continent, we find in the State of New Jersey a +series of sandy and argillaceous beds wholly unlike in mineral +character to our Upper Cretaceous system; which we can, +nevertheless, recognise as referable, palæontologically, to +the same division.</p> + +<p>That they were about the same age generally as the European +chalk and Neocomian, was the conclusion to which Dr. Morton and Mr. +Conrad came after their investigation of the fossils in 1834. The +strata consist chiefly of green sand and green marl, with an +overlying coralline limestone of a pale yellow colour, and the +fossils, on the whole, agree most nearly with those of the Upper +European series, from the Maestricht beds to the Gault inclusive. I +collected sixty shells from the New Jersey deposits in 1841, five +of which were identical with European species—<i>Ostrea larva, O. +vesicularis, Gryphæa costata, Pecten quinque-costatus, +Belemnitella mucronata.</i> As some of these have the greatest +vertical range in Europe, they might be expected more than any +others to recur in distant parts of the globe. Even where the +species were different, the generic forms, such as the Baculite and +certain sections of Ammonites, as also the <i>Inoceramus</i> (see +<a href="../images2/fig251.jpg">Fig. 252</a>) and other bivalves, have +a decidedly cretaceous aspect. Fifteen out of the sixty shells +above alluded to were regarded by Professor Forbes as good +geographical representatives of well-known cretaceous fossils of +Europe. The correspondence, therefore, is not small, when we +reflect that the part of the United States where these strata occur +is between 3000 and 4000 miles distant from the chalk of Central +and Northern Europe, and that there is a difference of ten degrees +in the latitude of the places compared on opposite sides of the +Atlantic. Fish of the genera <i>Lamna, Galeus,</i> and <i> +Carcharodon</i> are common to New Jersey and the European +cretaceous rocks. So also is the genus <i>Mosasaurus</i> among +reptiles.</p> + +<p>It appears from the labours of Dr. Newberry and others, that the +Cretaceous strata of the United States east and west of the +Appalachians are characterised by a flora decidedly analogous to +that of Aix-la-Chapelle above-mentioned, and therefore having +considerable resemblance to the vegetation of the Tertiary and +Recent Periods.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch16.html"> +Chapter XVI</a> / <a href="ch18.html">Chapter XVIII</a></small> +</body> +</html> + |