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diff --git a/old/3772-h/files/ch18.html b/old/3772-h/files/ch18.html new file mode 100644 index 0000000..b66517c --- /dev/null +++ b/old/3772-h/files/ch18.html @@ -0,0 +1,673 @@ +<!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 308">[ 308 ]</a></p> + +<p> </p> + +<center><b>Chapter XVIII</b><br> +<br> +LOWER CRETACEOUS OR NEOCOMIAN FORMATION.</center> + +<p class="intro">Classification of marine and fresh-water Strata. +— Upper Neocomian. — Folkestone and Hythe Beds. — +Atherfield Clay. — Similarity of Conditions causing +Reappearance of Species after short Intervals. — Upper +Speeton Clay. — Middle Neocomian. — Tealby Series. +— Middle Speeton Clay. — Lower Neocomian. — Lower +Speeton Clay. — Wealden Formation. — Fresh-water +Character of the Wealden. — Weald Clay. — Hastings +Sands. — Punfield Beds of Purbeck, Dorsetshire. — +Fossil Shells and Fish of the Wealden. — Area of the Wealden. +— Flora of the Wealden.</p> + +<p>We now come to the Lower Cretaceous Formation which was formerly +called Lower Greensand, and for which it will be useful for reasons +before explained (<a href="ch17.html#page 282">p. 282</a>) to use +the term “Neocomian."</p> + +<center><small>LOWER CRETACEOUS OR NEOCOMIAN +GROUP.</small></center> + +<table border="1" cellspacing="0" cellpadding="4" summary= +"Column 1: Marine; column 2: Fresh-water."> +<tr> +<td align="center">Marine</td> +<td align="center">Fresh-water</td> +</tr> + +<tr> +<td align="left"> +<ol> +<li>Upper Neocomian—Greensand of Folkestone, Sandgate, and Hythe, +Atherfield clay, upper part of Speeton clay.</li> + +<li>Middle Neocomian—Punfield Marine bed, Tealby beds, middle part +of Speeton clay.</li> + +<li>Lower Neocomian—Lower part of Speeton clay.</li> +</ol> +</td> +<td align="left" valign="middle">Part of Wealden beds of Kent, +Surrey, Sussex, Hants, and Dorset.</td> +</tr> +</table> + +<p>In Western France, the Alps, the Carpathians, Northern Italy, +and the Apennines, an extensive series of rocks has been described +by Continental geologists under the name of Tithonian. These beds, +which are without any marine equivalent in this country, appear +completely to bridge over the interval between the Neocomian and +the Oolites. They may, perhaps, as suggested by Mr. Judd, be of the +same age as part of the Wealden series.</p> + +<br> + + +<center><small>UPPER NEOCOMIAN.</small></center> + +<p><b>Folkstone and Hythe Beds.</b>—The sands which crop out +beneath the Gault in Wiltshire, Surrey, and Sussex are sometimes in +the uppermost part pure white, at others of a yellow and +ferruginous colour, and some of the beds contain much green matter. +At Folkestone they contain layers of calcareous matter and chert, +and at Hythe, in the neighbourhood, as also at Maidstone and other +parts of Kent, the limestone called Kentish Rag is intercalated. +This somewhat clayey</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 309">[ 309 ]</a></p> + +<p>and calcareous stone forms strata two feet thick, alternating +with quartzose sand. The total thickness of these Folkestone and +Hythe beds is less than 300 feet, and they are seen to rest +immediately on a grey clay, to which we shall presently allude as +the Atherfield clay. Among the fossils of the Folkestone and Hythe +beds we may mention <i>Nautilus plicatus</i> (Fig. 277), <i> +Ancyloceras (Scaphites) gigas</i> (Fig. 278), which has been aptly +described as an Ammonite more or less uncoiled; <i>Trigonia +caudata</i> (Fig. 280), <i>Gervillia anceps</i> (Fig. 279), a +bivalve genus allied to Avicula, and <i>Terebratula sella</i> (<a +href="../images2/fig281.jpg">Fig. 281</a>). In ferruginous beds of the +same age in Wiltshire is found a remarkable shell called <i>Diceras +Lonsdalii</i> (<a href="../images2/fig281.jpg">Fig. 282</a>), which +abounds in the Upper and Middle Neocomian of Southern Europe. This +genus is closely allied to Chama, and the cast of the interior has +been compared to the horns of a goat.</p> + +<center><img src="../images2/fig277.jpg" width="433" height="350" alt= +"Fig. 277: Nautilus licatus. Fig. 278: Ancyloceras gigas. Fig. 279: Gervillia anceps. Fig. 280: Trigonia caudata."> +</center> + +<p><b>Atherfield Clay.</b>—We mentioned before that the +Folkstone and Hythe series rest on a grey clay. This clay is only +of slight thickness in Kent and Surrey, but acquires great +dimensions at Atherfield, in the Isle of Wight. The difference, +indeed, in mineral character and thickness of the Upper Neocomian +formation near Folkestone, and the corresponding beds in the south +of the Isle of Wight, about</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 310">[ 310 ]</a></p> + +<center><img src="../images2/fig281.jpg" width="394" height="196" alt= +"Fig. 281: Terebratula sella. Fig. 282: Diceras Lonsdalii. a. The bivavle shell, b. Cast of one of the valves enlarged."> +</center> + +<p>100 miles distant, is truly remarkable. In the latter place we +find no limestone answering to the Kentish Rag, and the entire +thickness from the bottom of the Atherfield clay to the top of the +Neocomian, instead of being less than 300 feet as in Kent, is given +by the late Professor E. Forbes as 843 feet, which he divides into +sixty-three strata, forming three groups. The uppermost of these +consists of ferruginous sands, the second of sands and clay, and +the third or lowest of a brown clay, abounding in fossils.</p> + +<p>Pebbles of quartzose sandstone, jasper, and flinty slate, +together with grains of chlorite and mica, and, as Mr. +Godwin-Austen has shown, fragments and water-worn fossils of the +oolitic rocks, speak plainly of the nature of the pre-existing +formations, by the wearing down of which the Neocomian beds were +formed. The land, consisting of such rocks, was doubtless submerged +before the origin of the white chalk, a deposit which was formed in +a more open sea, and in clearer waters.</p> + +<img src="../images2/fig283.jpg" width="234" height="206" alt= +"Fig. 283: Perna mulleti." align="left"> + +<p>Among the shells of the Atherfield clay the biggest and most +abundant shell is the large <i>Perna Mulleti,</i> of which a +reduced figure is given in Fig. 283.</p> + +<p><i>Similarity of Conditions causing Reappearance of +Species.</i>—Some species of mollusca and other fossils range +through the whole series, while others are confined to particular +subdivisions, and Forbes laid down a law which has since been found +of very general application in regard to estimating the +chronological relations of consecutive</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 311">[ 311 ]</a></p> + +<p>strata. Whenever similar conditions, he says, are repeated, the +same species reappear, provided too great a lapse of time has not +intervened; whereas if the length of the interval has been +geologically great, the same genera will reappear represented by +distinct species. Changes of depth, or of the mineral nature of the +sea-bottom, the presence or absence of lime or of peroxide of iron, +the occurrence of a muddy, or a sandy, or a gravelly bottom, are +marked by the banishment of certain species and the predominance of +others. But these differences of conditions being mineral, +chemical, and local in their nature, have no necessary connection +with the extinction, throughout a large area, of certain animals or +plants. When the forms proper to loose sand or soft clay, or to +perfectly clear water, or to a sea of moderate or great depth, +recur with all the same species, we may infer that the interval of +time has been, geologically speaking, small, however dense the mass +of matter accumulated. But if, the genera remaining the same, the +species are changed, we have entered upon a new period; and no +similarity of climate, or of geographical and local conditions, can +then recall the old species which a long series of destructive +causes in the animate and inanimate world has gradually +annihilated.</p> + +<img src="../images2/fig284.jpg" width="160" height="167" alt= +"Fig. 284: Ammonites Deshayesii." align="right"> + +<p><b>Speeton Clay, Upper Division.</b>—On the coast, beneath +the white chalk of Flamborough Head, in Yorkshire, an argillaceous +formation crops out, called the Speeton clay, several hundred feet +in thickness, the palæontological relations of which have +been ably worked out by Mr. John W. Judd,* and he has shown that it +is separable into three divisions, the uppermost of which, 150 feet +thick, and containing 87 species of mollusca, decidedly belongs to +the Atherfield clay and associated strata of Hythe and Folkestone, +already described. It is characterised by the <i>Perna Mulleti</i> +(<a href="../images2/fig283.jpg">Fig. 283</a>) and <i>Terebratula +sella</i> (<a href="../images2/fig281.jpg">Fig. 281</a>), and by <i> +Ammonites Deshayesii</i> (Fig. 284), a well-known Hythe fossil. +Fine skeletons of reptiles of the genera Pliosaurus and Teleosaurus +have been obtained from this clay. At the base of this upper +division of the Speeton clay there occurs a layer of large +Septaria, formerly worked for the manufacture of cement. This bed +is crowded with fossils, especially Ammonites, one species of +which, three feet in diameter, was observed by Mr. Judd.</p> + +<p class="fnote">* Judd, Speeton clay, Quart. Geol. Journ., vol. +xxiv, 1868, p. 218.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 312">[ 312 ]</a></p> + +<center><small>MIDDLE NEOCOMIAN.</small></center> + +<p><b>Tealby Series.</b>—At Tealby, a village in the +Lincolnshire Wolds, there crop out beneath the white chalk some +non-fossiliferous ferruginous sands about twenty-feet thick, +beneath which are beds of clay and limestone, about fifty feet +thick, with an interesting suite of fossils, among which are <i> +Pecten cinctus</i> (Fig. 285), from 9 to 12 inches in diameter, <i> +Ancyloceras Duvallei</i> (Fig. 286), and some forty other shells, +many of them common to the Middle Speeton clay, about to be +mentioned. Mr. Judd remarks that as <i>Ammonites clypeiformis</i> +and <i>Terebratula hippopus</i> characterise the Middle Neocomian +of the Continent, it is to this stage that the Tealby series +containing the same fossils may be assigned.*</p> + +<center><img src="../images2/fig285.jpg" width="394" height="229" alt= +"Fig. 285: Pecten cinctus. Fig. 286: Ancyloceras (Crioceras) Duvallei."> +</center> + +<p>The middle division of the Speeton clay, occurring at Speeton +below the cement-bed, before alluded to, is 150 feet thick, and +contains about 39 species of mollusca, half of which are common to +the overlying clay. Among the peculiar shells, <i>Pecten +cinctus</i> (Fig. 285) and <i>Ancyloceras (Crioceras) Duvallei</i> +(Fig. 286) occur.</p> + +<br> + + +<center><small>LOWER NEOCOMIAN.</small></center> + +<p>In the lower division of the Speeton clay, 200 feet thick, 46 +species of mollusca have been found, and three divisions, each +characterised by its peculiar ammonite, have been noticed by Mr. +Judd. The central zone is marked by <i>Ammonites Noricus</i> (see +Fig. 287). On the Continent these beds are well-known by their +corresponding fossils, the Hils clay and conglomerate of the north +of Germany agreeing with</p> + +<p class="fnote">* Judd, Quart. Geol. Journ., 1867, vol. xxiii, p. +249.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 313">[ 313 ]</a></p> + +<img src="../images2/fig287.jpg" width="155" height="174" alt= +"Fig. 287: Ammonites Noricus." align="right"> + +<p>the Middle and Lower Speeton, the latter of which, with the same +mineral characters and fossils as in Yorkshire, is also found in +the little island of Heligoland. Yellow limestone, which I have +myself seen near Neuchatel, in Switzerland, represents the Lower +Neocomian at Speeton.</p> + +<br> +<center><small>WEALDEN FORMATION.</small></center> + +<p>Beneath the Atherfield clay or Upper Neocomian of the S.E. of +England, a fresh-water formation is found, called the Wealden, +which, although it occupies a small horizontal area in Europe, as +compared to the White Chalk and the marine Neocomian beds, is +nevertheless of great geological interest, since the imbedded +remains give us some insight into the nature of the terrestrial +fauna and flora of the Lower Cretaceous epoch. The name of Wealden +was given to this group because it was first studied in parts of +Kent, Surrey, and Sussex, called the Weald; and we are indebted to +Dr. Mantell for having shown, in 1822, in his “Geology of +Sussex,” that the whole group was of fluviatile origin. In +proof of this he called attention to the entire absence of +Ammonites, Belemnites, Brachiopoda, Echinodermata, Corals, and +other marine fossils, so characteristic of the Cretaceous rocks +above, and of the Oolitic strata below, and to the presence in the +Weald of Paludinæ, Melaniæ, Cyrenæ, and various +fluviatile shells, as well as the bones of terrestrial reptiles and +the trunks and leaves of land-plants.</p> + +<p>The evidence of so unexpected a fact as that of a dense mass of +purely fresh-water origin underlying a deep-sea deposit (a +phenomenon with which we have since become familiar) was received, +at first, with no small doubt and incredulity. But the relative +position of the beds is unequivocal; the Weald Clay being +distinctly seen to pass beneath the Atherfield Clay in various +parts of Surrey, Kent, and Sussex, and to reappear in the Isle of +Wight at the base of the Cretaceous series, being, no doubt, +continuous far beneath the surface, as indicated by the dotted +lines in <a href="../images2/fig288.jpg">Fig. 288.</a> They are also +found occupying the same relative position below the chalk in the +peninsula of Purbeck, Dorsetshire, where, as we shall see in the +sequel, they repose on strata referable to the Upper Oolite.</p> + +<p><i>Weald Clay.</i>—The Upper division, or Weald Clay, is, +in great part, of fresh-water origin, but in its highest +portion</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 314">[ 314 ]</a></p> + +<center><img src="../images2/fig288.jpg" width="410" height="159" alt= +"Fig. 288"></center> + +<p>contains beds of oysters and other marine shells which indicate +fluvio-marine conditions. The uppermost beds are not only +conformable, as Dr. Fitton observes, to the inferior strata of the +overlying Neocomian, but of similar mineral composition. To explain +this, we may suppose that, as the delta of a great river was +tranquilly subsiding, so as to allow the sea to encroach upon the +space previously occupied by fresh-water, the river still continued +to carry down the same sediment into the sea. In confirmation of +this view it may be stated that the remains of the <i>Iguanodon +Mantelli,</i> a gigantic terrestrial reptile, very characteristic +of the Wealden, has been discovered near Maidstone, in the +overlying Kentish Rag, or marine limestone of the Upper Neocomian. +Hence we may infer that some of the saurians which inhabited the +country of the great river continued to live when part of the +district had become submerged beneath the sea. Thus, in our own +times, we may suppose the bones of large alligators to be +frequently entombed in recent fresh-water strata in the delta of +the Ganges. But if part of that delta should sink down so as to be +covered by the sea, marine formations might begin to accumulate in +the same space where fresh-water beds had previously been formed; +and yet the Ganges might still pour down its turbid waters in the +same direction, and carry seaward the carcasses of the same species +of alligator, in which case their bones might be included in marine +as well as in subjacent fresh-water strata.</p> + +<p>The Iguanodon, first discovered by Dr. Mantell, was an +herbivorous reptile, of which the teeth, though bearing a great +analogy, in their general form and crenated edges (see <a href= +"../images2/fig289.jpg">Figs. 289</a> <i>a</i> and <i>b</i>), to the +modern Iguanas which now frequent the tropical woods of America and +the West Indies, exhibit many important differences. It appears +that they have often been worn by the process of mastication; +whereas the existing herbivorous reptiles clip and gnaw off the +vegetable productions on which they feed, but do not chew them. +Their teeth frequently present an appearance</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 315">[ 315 ]</a></p> + +<center><img src="../images2/fig289.jpg" width="388" height="318" alt= +"Fig. 289 a, b: Tooth of Iguanodon Mantelli. Fig. 290: a. Partially worn tooth of young individual of the same; <i>b.</i> Crown of tooth in adult worn down."> +</center> + +<p>of having been chipped off, but never, like the fossil teeth of +the Iguanodon, have a flat ground surface (see Fig. 290, <i>b</i>) +resembling the grinders of herbivorous mammalia. Dr. Mantell +computes that the teeth and bones of this species which passed +under his examination during twenty years must have belonged to no +less than seventy-one distinct individuals, varying in age and +magnitude from the reptile just burst from the egg, to one of which +the femur measured twenty-four inches in circumference. Yet, +notwithstanding that the teeth were more numerous than any other +bones, it is remarkable that it was not until the relics of all +these individuals had been found, that a solitary example of part +of a jaw-bone was obtained. Soon afterwards remains both of the +upper and lower jaw were met with in the Hastings beds in Tilgate +Forest, near Cuckfield. In the same sands at Hastings, Mr. Beckles +found large tridactyle impressions which it is conjectured were +made by the hind feet of this animal, on which it is ascertained +that there were only three well-developed toes.</p> + +<img src="../images2/fig291.jpg" width="86" height="125" alt= +"Fig. 291: Cypris spinigera." align="right"> + +<p>Occasionally bands of limestone, called Sussex Marble, occur in +the Weald Clay, almost entirely composed of a species of <i> +Paludina,</i> closely resembling the common <i>P. vivipara</i> of +English rivers. Shells of the <i>Cypris,</i> a genus of Crustaceans +mentioned (<a href="ch3.html#page 57">p. 57</a>) as abounding in +lakes and ponds, are also plentifully scattered through the clays +of the Wealden,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 316">[ 316 ]</a></p> + +<img src="../images2/fig292.jpg" width="141" height="117" alt= +"Fig. 292: Weald clay with Cyprides." align="left"> + +<p>sometimes producing, like plates of mica, a thin lamination (see +Fig. 292).</p> + +<p><b>Hastings Sands.</b>—This lower division of the Wealden +consists of sand, sandstone, calciferous grit, clay, and shale; the +argillaceous strata, notwithstanding the name, predominating +somewhat over the arenaceous, as will be seen by reference to the +following table, drawn up by Messrs. Drew and Foster, of the +Geological Survey of Great Britain:</p> + +<center> +<table border="1" cellpadding="4" cellspacing="0" summary= +"Names of subordinate formations, Mineral composition of the strata, and Thickness in feet of the Hastings Sand."> +<tr> +<td> </td> +<td align="center">Names of Subordinate<br> +Formations.</td> +<td align="center">Mineral Composition<br> +of the Strata.</td> +<td align="center">Thickness<br> +in feet.</td> +</tr> + +<tr> +<td valign="middle" align="left" rowspan="4">Hastings Sand</td> +<td align="left" valign="top">Tunbridge Wells Sand</td> +<td align="left" valign="top">Sandstone and loam</td> +<td align="center" valign="top">150</td> +</tr> + +<tr> +<td align="left" valign="top">Wadhurst Clay</td> +<td align="left" valign="top">Blue and brown shale and clay, +with<br> +a little calc-grit</td> +<td align="center" valign="top">100</td> +</tr> + +<tr> +<td align="left" valign="top">Ashdown Sand</td> +<td align="left" valign="top">Hard sand, with some beds of +calc-grit</td> +<td align="center" valign="top">160</td> +</tr> + +<tr> +<td align="left" valign="top">Ashburnham Beds</td> +<td align="left" valign="top">Mottled white and red clay, with<br> +some sandstone</td> +<td align="center" valign="top">330</td> +</tr> +</table> +</center> + +<p>The picturesque scenery of the “High Rocks” and +other places in the neighbourhood of Tunbridge Wells is caused by +the steep natural cliffs, to which a hard bed of white sand, +occurring in the upper part of the Tunbridge Wells Sand, mentioned +in the above table, gives rise. This bed of “rock-sand” +varies in thickness from 25 to 48 feet. Large masses of it, which +were by no means hard or capable of making a good building-stone, +form, nevertheless, projecting rocks with perpendicular faces, and +resist the degrading action of the river because, says Mr. Drew, +they present a solid mass without planes of division. The +calcareous sandstone and grit of Tilgate Forest, near Cuckfield, in +which the remains of the Iguanodon and Hylæosaurus were first +found by Dr. Mantell, constitute an upper member of the Tunbridge +Wells Sand, while the “sand-rock” of the Hastings +cliffs, about 100 feet thick, is one of the lower members of the +same. The reptiles, which are very abundant in this division, +consist partly of saurians, referred by Owen and Mantell to eight +genera, among which, besides those already enumerated, we find the +Megalosaurus and Plesiosaurus. The Pterodactyl also, a flying +reptile, is met with in the same strata, and many remains of +Chelonians of the genera <i>Trionyx</i> and <i>Emys,</i> now +confined to tropical regions.</p> + +<p>The fishes of the Wealden are chiefly referable to the Ganoid +and Placoid orders. Among them the teeth and scales of <i> +Lepidotus</i> are most widely diffused (see Fig. 293, next page). +These</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 317">[ 317 ]</a></p> + +<center><img src="../images2/fig293.jpg" width="331" height="172" alt= +"Fig. 293: Lepidotus Mantelli, a. Palate and teeth, b. Side view of teeth, c. Scale."> +</center> + +<img src="../images2/fig294.jpg" width="257" height="460" alt= +"Fig. 294: Unio Valdensis. Fig. 295: Under side of slab of sandstone about one yard in diameter." + align="left"> + +<p>ganoids were allied to the <i>Lepidosteus,</i> or Gar-pike, of +the American rivers. The whole body was covered with large +rhomboidal scales, very thick, and having the exposed part coated +with enamel. Most of the species of this genus are supposed to have +been either river-fish, or inhabitants of the sea at the mouth of +estuaries.</p> + +<p>At different heights in the Hastings Sands, we find again and +again slabs of sandstone with a strong ripple-mark, and between +these slabs beds of clay many yards thick. In some places, as at +Stammerham, Horsham, near there, are indications of this clay +having been exposed so as to dry and crack before the next layer +was thrown down upon it. The open cracks in the clay have served as +moulds, of which casts have been taken in relief, and which are, +therefore, seen on the lower surface of the sandstone (see Fig. +295).</p> + +<p>Near the same place a reddish sandstone occurs in which</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 318">[ 318 ]</a></p> + +<img src="../images2/fig296.jpg" width="185" height="203" alt= +"Fig. 296: Sphenopteris gracilis." align="left"> + +<p>are innumerable traces of a fossil vegetable, apparently <i> +Sphenopteris,</i> the stems and branches of which are disposed as +if the plants were standing erect on the spot where they originally +grew, the sand having been gently deposited upon and around them; +and similar appearances have been remarked in other places in this +formation.* In the same division also of the Wealden, at Cuckfield, +is a bed of gravel or conglomerate, consisting of water-worn +pebbles of quartz and jasper, with rolled bones of reptiles. These +must have been drifted by a current, probably in water of no great +depth.</p> + +<p>From such facts we may infer that, notwithstanding the great +thickness of this division of the Wealden, the whole of it was a +deposit in water of a moderate depth, and often extremely shallow. +This idea may seem startling at first, yet such would be the +natural consequence of a gradual and continuous sinking of the +ground in an estuary or bay, into which a great river discharged +its turbid waters. By each foot of subsidence, the fundamental rock +would be depressed one foot farther from the surface; but the bay +would not be deepened, if newly-deposited mud and sand should raise +the bottom one foot. On the contrary, such new strata of sand and +mud might be frequently laid dry at low water, or overgrown for a +season by a vegetation proper to marshes.</p> + +<p><b>Punfield Beds, Brackish and Marine.</b>—The shells of +the Wealden beds belong to the genera <i>Melanopsis, Melania, +Paludina, Cyrena, Cyclas, Unio</i> (see <a href= +"../images2/fig294.jpg">Fig. 294</a>), and others, which inhabit +rivers or lakes; but one band has been found at Punfield, in +Dorsetshire, indicating a brackish state of the water, where the +genera <i>Corbula, Mytilus,</i> and <i>Ostrea</i> occur; and in +some places this bed becomes purely marine, containing some +well-known Neocomian fossils, among which <i>Ammonites +Deshayesii</i> (<a href="../images2/fig284.jpg">Fig. 284</a>) may be +mentioned. Others are peculiar as British, but very characteristic +of the Upper and Middle Neocomian of Spain, and among these the <i> +Vicarya Lujani</i> (<a href="../images2/fig297.jpg">Fig. 297</a>), a +shell allied to Nerinea, is conspicuous.</p> + +<p>By reference to table (<a href="#page 308">p. 308</a>) it will +be seen that the</p> + +<p class="fnote">* Mantell, Geol. of S.E. of England, p. 244.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 319">[ 319 ]</a></p> + +<img src="../images2/fig297.jpg" width="160" height="237" alt= +"Fig. 297: Vicarya Lujani." align="right"> + +<p>Wealden beds are given as the fresh-water equivalents of the +Marine Neocomian. The highest part of them in England may, for +reasons just given, be regarded as Upper Neocomian, while some of +the inferior portions may correspond in age to the Middle and Lower +divisions of that group. In favour of this latter view, M. Marcou +mentions that a fish called <i>Asteracanthus granulosus,</i> +occurring in the Tilgate beds, is characteristic of the lowest beds +of the Neocomian of the Jura, and it is well known that <i>Corbula +alata,</i> common in the Ashburnham beds, is found also at the base +of the Neocomian of the Continent.</p> + +<p><i>Area of the Wealden.</i>—In regard to the geographical +extent of the Wealden, it can not be accurately laid down, because +so much of it is concealed beneath the newer marine formations. It +has been traced about 320 English miles from west to east, from the +coast of Dorsetshire to near Boulogne, in France; and nearly 200 +miles from north-west to south-east, from Surrey and Hampshire to +Vassy, in France. If the formation be continuous throughout this +space, which is very doubtful, it does not follow that the whole +was contemporaneous; because, in all likelihood, the physical +geography of the region underwent frequent changes throughout the +whole period, and the estuary may have altered its form, and even +shifted its place. Dr. Dunker, of Cassel, and H. von Meyer, in an +excellent monograph on the Wealdens of Hanover and Westphalia, have +shown that they correspond so closely, not only in their fossils, +but also in their mineral characters, with the English series, that +we can scarcely hesitate to refer the whole to one great delta. +Even then, the magnitude of the deposit may not exceed that of many +modern rivers. Thus, the delta of the Quorra or Niger, in Africa, +stretches into the interior for more than 170 miles, and occupies, +it is supposed, a space of more than 300 miles along the coast, +thus forming a surface of more than 25,000 square miles, or equal +to about one-half of England.† Besides, we know not, in such +cases, how far the fluviatile sediment and organic remains of the +river and the land may be carried out from the coast, and spread +over the bed of the sea. I have</p> + +<p class="fnote">* Foss. de Utrillas.<br> +† Fitton, Geol. of Hastings, p. 58, who cites Lander’s +Travels.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 320">[ 320 ]</a></p> + +<p>shown, when treating of the Mississippi, that a more ancient +delta, including species of shells such as now inhabit Louisiana, +has been upraised, and made to occupy a wide geographical area, +while a newer delta is forming; and the possibility of such +movements and their effects must not be lost sight of when we +speculate on the origin of the Wealden.</p> + +<p>It may be asked where the continent was placed, from the ruins +of which the Wealden strata were derived, and by the drainage of +which a great river was fed. If the Wealden was gradually going +downward 1000 feet or more perpendicularly, a large body of +fresh-water would not continue to be poured into the sea at the +same point. The adjoining land, if it participated in the movement, +could not escape being submerged. But we may suppose such land to +have been stationary, or even undergoing contemporaneous slow +upheaval. There may have been an ascending movement in one region, +and a descending one in a contiguous parallel zone of country. But +even if that were the case, it is clear that finally an extensive +depression took place in that part of Europe where the deep sea of +the Cretaceous period was afterwards brought in.</p> + +<p><i>Thickness of the Wealden.</i>—In the Weald area itself, +between the North and South Downs, fresh-water beds to the +thickness of 1600 feet are known, the base not being reached. +Probably the thickness of the whole Wealden series, as seen in +Swanage Bay, can not be estimated as less than 2000 feet.</p> + +<p><i>Wealden Flora.</i>—The flora of the Wealden is +characterised by a great abundance of Coniferæ, +Cycadeæ, and Ferns, and by the absence of leaves and fruits +of Dicotyledonous Angiosperms. The discovery in 1855, in the +Hastings beds of the Isle of Wight, of Gyrogonites, or +spore-vessels of the Chara, was the first example of that genus of +plants, so common in the tertiary strata, being found in a +Secondary or Mesozoic rock.</p> + +<p class="fnote">* See <a href="ch6.html#page 102">p. 102</a> and +Second Visit to the United States, vol. ii, chap. xxxiv.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch17.html"> +Chapter XVII</a> / <a href="ch19.html">Chapter XIX</a></small> +</body> +</html> + |