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diff --git a/old/3772-h/files/ch2.html b/old/3772-h/files/ch2.html new file mode 100644 index 0000000..4ba4a7c --- /dev/null +++ b/old/3772-h/files/ch2.html @@ -0,0 +1,623 @@ +<!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 35">[ 35 ]</a></p> + +<p> </p> + +<center><b>Chapter II</b><br> +<br> + AQUEOUS ROCKS.—THEIR COMPOSITION AND FORMS OF +STRATIFICATION.</center> + +<p class="intro">Mineral Composition of Strata. — Siliceous +Rocks. — Argillaceous. — Calcareous. — Gypsum. — Forms of Stratification. — Original Horizontality. — Thinning +out. — Diagonal Arrangement. — Ripple-mark.</p> + +<p>In pursuance of the arrangement explained in the last chapter, +we shall begin by examining the aqueous or sedimentary rocks, which +are for the most part distinctly stratified, and contain fossils. +We may first study them with reference to their mineral +composition, external appearance, position, mode of origin, organic +contents, and other characters which belong to them as aqueous +formations, independently of their age, and we may afterwards +consider them chronologically or with reference to the successive +geological periods when they originated.</p> + +<p>I have already given an outline of the data which led to the +belief that the stratified and fossiliferous rocks were originally +deposited under water; but, before entering into a more detailed +investigation, it will be desirable to say something of the +ordinary materials of which such strata are composed. These may be +said to belong principally to three divisions, the siliceous, the +argillaceous, and the calcareous, which are formed respectively of +flint, clay, and carbonate of lime. Of these, the siliceous are +chiefly made up of sand or flinty grains; the argillaceous, or +clayey, of a mixture of siliceous matter with a certain proportion, +about a fourth in weight, of aluminous earth; and, lastly, the +calcareous rocks, or limestones, of carbonic acid and lime.</p> + +<p> +<b>Siliceous and Arenaceous +Rocks.</b>—To speak first of the sandy division: beds +of loose sand are frequently met with, of which the grains consist +entirely of silex, which term comprehends all purely siliceous +minerals, as quartz and common flint. Quartz is silex in its purest +form. Flint usually contains some admixture of alumina and oxide of +iron. The siliceous grains in sand are usually rounded, as if by +the action of running water. Sandstone is an aggregate of such +grains, which often cohere together without any visible cement, but +more commonly are bound together</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 36">[ 36 ]</a></p> + +<br> + + +<p>by a slight quantity of siliceous or calcareous matter, or by +oxide of iron or clay.</p> + +<p>Pure siliceous rocks may be known by not effervescing when a +drop of nitric, sulphuric or other acid is applied to them, or by +the grains not being readily scratched or broken by ordinary +pressure. In nature there is every intermediate gradation, from +perfectly loose sand to the hardest sandstone. In <i>micaceous +sandstones</i> mica is very abundant; and the thin silvery plates +into which that mineral divides are often arranged in layers +parallel to the planes of stratification, giving a slaty or +laminated texture to the rock.</p> + +<p>When sandstone is coarse-grained, it is usually called <i> +grit.</i> If the grains are rounded, and large enough to be called +pebbles, it becomes a <i>conglomerate</i> or <i>pudding-stone,</i> +which may consist of pieces of one or of many different kinds of +rock. A conglomerate, therefore, is simply gravel bound together by +cement.</p> + +<p><b>Argillaceous Rocks.</b>—Clay, +strictly speaking, is a mixture of silex or flint with a large +proportion, usually about one fourth, of alumina, or argil; but in +common language, any earth which possesses sufficient ductility, +when kneaded up with water, to be fashioned like paste by the hand, +or by the potter’s lathe, is called a <i>clay</i>; and such clays +vary greatly in their composition, and are, in general, nothing +more than mud derived from the decomposition or wearing down of +rocks. The purest clay found in nature is porcelain clay, or +kaolin, which results from the decomposition of a rock composed of +feldspar and quartz, and it is almost always mixed with quartz. The +kaolin of China consists of 71·15 parts of silex, +15·86 of alumine, 1·92 of lime, and 6·73 of +water;* but other porcelain clays differ materially, that of +Cornwall being composed, according to Boase, of nearly equal parts +of silica and alumine, with 1 per cent of magnesia.† <i> +Shale</i> has also the property, like clay, of becoming plastic in +water: it is a more solid form of clay, or argillaceous matter, +condensed by pressure. It always divides into laminæ more or +less regular.</p> + +<p>One general character of all argillaceous rocks is to give out a +peculiar, earthy odour when breathed upon, which is a test of the +presence of alumine, although it does not belong to pure alumine, +but, apparently, to the combination of that substance with oxide of +iron.‡</p> + +<p><b>Calcareous Rocks.</b>—This +division comprehends those rocks which, like chalk, are composed +chiefly of lime and carbonic</p> + +<p class="fnote">* W. Phillips, Mineralogy, p.33.<br> +† Phil. Mag., vol. x, 1837.<br> +‡ See W. Phillips’s Mineralogy, “Alumine.”</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 37">[ 37 ]</a></p> + +<br> + + +<p>acid. Shells and corals are also formed of the same elements, +with the addition of animal matter. To obtain pure lime it is +necessary to calcine these calcareous substances, that is to say, +to expose them to heat of sufficient intensity to drive off the +carbonic acid, and other volatile matter. White chalk is sometimes +pure carbonate of lime; and this rock, although usually in a soft +and earthy state, is occasionally sufficiently solid to be used for +building, and even passes into a <i>compact</i> stone, or a stone +of which the separate parts are so minute as not to be +distinguishable from each other by the naked eye.</p> + +<p>Many limestones are made up entirely of minute fragments of +shells and coral, or of calcareous sand cemented together. These +last might be called “calcareous sandstones;” but that term is more +properly applied to a rock in which the grains are partly +calcareous and partly siliceous, or to quartzose sandstones, having +a cement of carbonate of lime.</p> + +<p>The variety of limestone called <i>oolite</i> is composed of +numerous small egg-like grains, resembling the roe of a fish, each +of which has usually a small fragment of sand as a nucleus, around +which concentric layers of calcareous matter have accumulated.</p> + +<p>Any limestone which is sufficiently hard to take a fine polish +is called <i>marble.</i> Many of these are fossiliferous; but +statuary marble, which is also called saccharoid limestone, as +having a texture resembling that of loaf-sugar, is devoid of +fossils, and is in many cases a member of the metamorphic +series.</p> + +<p><i>Siliceous limestone</i> is an intimate mixture of carbonate +of lime and flint, and is harder in proportion as the flinty matter +predominates.</p> + +<p>The presence of carbonate of lime in a rock may be ascertained +by applying to the surface a small drop of diluted sulphuric, +nitric, or muriatic acid, or strong vinegar; for the lime, having a +greater chemical affinity for any one of these acids than for the +carbonic, unites immediately with them to form new compounds, +thereby becoming a sulphate, nitrate or muriate of lime. The +carbonic acid, when thus liberated from its union with the lime, +escapes in a gaseous form, and froths up or effervesces as it makes +its way in small bubbles through the drop of liquid. This +effervescence is brisk or feeble in proportion as the limestone is +pure or impure, or, in other words, according to the quantity of +foreign matter mixed with the carbonate of lime. Without the aid of +this test, the most experienced eye can not always detect the +presence of carbonate of lime in rocks.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 38">[ 38 ]</a></p> + +<br> + + +<p>The above-mentioned three classes of rocks, the siliceous, +argillaceous, and calcareous, pass continually into each other, and +rarely occur in a perfectly separate and pure form. Thus it is an +exception to the general rule to meet with a limestone as pure as +ordinary white chalk, or with clay as aluminous as that used in +Cornwall for porcelain, or with sand so entirely composed of +siliceous grains as the white sand of Alum Bay, in the Isle of +Wight, employed in the manufacture of glass, or sandstone so pure +as the grit of Fontainebleau, used for pavement in France. More +commonly we find sand and clay, or clay and marl, intermixed in the +same mass. When the sand and clay are each in considerable +quantity, the mixture is called <i>loam.</i> If there is much +calcareous matter in clay it is called <i>marl</i>; but this term +has unfortunately been used so vaguely, as often to be very +ambiguous. It has been applied to substances in which there is no +lime; as, to that red loam usually called red marl in certain parts +of England. Agriculturists were in the habit of calling any soil a +marl which, like true marl, fell to pieces readily on exposure to +the air. Hence arose the confusion of using this name for soils +which, consisting of loam, were easily worked by the plough, though +devoid of lime.</p> + +<p><i>Marl slate</i> bears the same relation to marl which shale +bears to clay, being a calcareous shale. It is very abundant in +some countries, as in the Swiss Alps. Argillaceous or marly +limestone is also of common occurrence.</p> + +<p>There are few other kinds of rock which enter so largely into +the composition of sedimentary strata as to make it necessary to +dwell here on their characters. I may, however, mention two +others—magnesian limestone or dolomite, and gypsum. <i>Magnesian +limestone</i> is composed of carbonate of lime and carbonate of +magnesia; the proportion of the latter amounting in some cases to +nearly one half. It effervesces much more slowly and feebly with +acids than common limestone. In England this rock is generally of a +yellowish colour; but it varies greatly in mineralogical character, +passing from an earthy state to a white compact stone of great +hardness. <i>Dolomite,</i> so common in many parts of Germany and +France, is also a variety of magnesian limestone, usually of a +granular texture.</p> + +<p><i>Gypsum</i> is a rock composed of sulphuric acid, lime, and +water. It is usually a soft whitish-yellow rock, with a texture +resembling that of loaf-sugar, but sometimes it is entirely +composed of lenticular crystals. It is insoluble in acids, and does +not effervesce like chalk and dolomite, because it does not contain +carbonic acid gas, or fixed air, the lime</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 39">[ 39 ]</a></p> + +<br> + + +<p>being already combined with sulphuric acid, for which it has a +stronger affinity than for any other. Anhydrous gypsum is a rare +variety, into which water does not enter as a component part. <i> +Gypseous marl</i> is a mixture of gypsum and marl. <i>Alabaster</i> +is a granular and compact variety of gypsum found in masses large +enough to be used in sculpture and architecture. It is sometimes a +pure snow-white substance, as that of Volterra in Tuscany, well +known as being carved for works of art in Florence and Leghorn. It +is a softer stone than marble, and more easily wrought.</p> + +<p><b>Forms of +Stratification.</b>—A series of strata sometimes +consists of one of the above rocks, sometimes of two or more in +alternating beds.</p> + +<p>Thus, in the coal districts of England, for example, we often +pass through several beds of sandstone, some of finer, others of +coarser grain, some white, others of a dark colour, and below +these, layers of shale and sandstone or beds of shale, divisible +into leaf-like laminæ, and containing beautiful impressions +of plants. Then again we meet with beds of pure and impure coal, +alternating with shales and sandstones, and underneath the whole, +perhaps, are calcareous strata, or beds of limestone, filled with +corals and marine shells, each bed distinguishable from another by +certain fossils, or by the abundance of particular species of +shells or zoophytes.</p> + +<p>This alternation of different kinds of rock produces the most +distinct stratification; and we often find beds of limestone and +marl, conglomerate and sandstone, sand and clay, recurring again +and again, in nearly regular order, throughout a series of many +hundred strata. The causes which may produce these phenomena are +various, and have been fully discussed in my treatise on the modern +changes of the earth’s surface.* It is there seen that rivers +flowing into lakes and seas are charged with sediment, varying in +quantity, composition, colour, and grain according to the seasons; +the waters are sometimes flooded and rapid, at other periods low +and feeble; different tributaries, also, draining peculiar +countries and soils, and therefore charged with peculiar sediment, +are swollen at distinct periods. It was also shown that the waves +of the sea and currents undermine the cliffs during wintry storms, +and sweep away the materials into the deep, after which a season of +tranquillity succeeds, when nothing but the finest mud is spread by +the movements of the ocean over the same submarine area.</p> + +<p>It is not the object of the present work to give a +description</p> + +<p class="fnote">* Consult Index to Principles of Geology, +“Stratification,” “Currents,” “Deltas,” “Water,” etc.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 40">[ 40 ]</a></p> + +<br> + + +<p>of these operations, repeated as they are, year after year, and +century after century; but I may suggest an explanation of the +manner in which some micaceous sandstones have originated, namely, +those in which we see innumerable thin layers of mica dividing +layers of fine quartzose sand. I observed the same arrangement of +materials in recent mud deposited in the estuary of Laroche St. +Bernard in Brittany, at the mouth of the Loire. The surrounding +rocks are of gneiss, which, by its waste, supplies the mud: when +this dries at low water, it is found to consist of brown laminated +clay, divided by thin seams of mica. The separation of the mica in +this case, or in that of micaceous sandstones, may be thus +understood. If we take a handful of quartzose sand, mixed with +mica, and throw it into a clear running stream, we see the +materials immediately sorted by the water, the grains of quartz +falling almost directly to the bottom, while the plates of mica +take a much longer time to reach the bottom, and are carried +farther down the stream. At the first instant the water is turbid, +but immediately after the flat surfaces of the plates of mica are +seen all alone, reflecting a silvery light, as they descend slowly, +to form a distinct micaceous lamina. The mica is the heavier +mineral of the two; but it remains a longer time suspended in the +fluid, owing to its greater extent of surface. It is easy, +therefore, to perceive that where such mud is acted upon by a river +or tidal current, the thin plates of mica will be carried farther, +and not deposited in the same places as the grains of quartz; and +since the force and velocity of the stream varies from time to +time, layers of mica or of sand will be thrown down successively on +the same area.</p> + +<p><b>Original +Horizontality.</b>—It is said generally that the upper +and under surfaces of strata, or the “planes of stratification,” +are parallel. Although this is not strictly true, they make an +approach to parallelism, for the same reason that sediment is +usually deposited at first in nearly horizontal layers. Such an +arrangement can by no means be attributed to an original evenness +or horizontality in the bed of the sea: for it is ascertained that +in those places where no matter has been recently deposited, the +bottom of the ocean is often as uneven as that of the dry land, +having in like manner its hills, valleys, and ravines. Yet if the +sea should go down, or be removed from near the mouth of a large +river where a delta has been forming, we should see extensive +plains of mud and sand laid dry, which, to the eye, would appear +perfectly level, although, in reality, they would slope gently from +the land towards the sea.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 41">[ 41 ]</a></p> + +<br> + + +<p>This tendency in newly-formed strata to assume a horizontal +position arises principally from the motion of the water, which +forces along particles of sand or mud at the bottom, and causes +them to settle in hollows or depressions where they are less +exposed to the force of a current than when they are resting on +elevated points. The velocity of the current and the motion of the +superficial waves diminish from the surface downward, and are least +in those depressions where the water is deepest.</p> + +<img src="../images/fig1.jpg" width="175" height="73" align="right" +alt="Fig. 1"> + +<p>A good illustration of the principle here alluded to may be +sometimes seen in the neighbourhood of a volcano, when a section, +whether natural or artificial, has laid open to view a succession +of various-coloured layers of sand and ashes, which have fallen in +showers upon uneven ground. Thus let A B (Fig. 1) be two ridges, +with an intervening valley. These original inequalities of the +surface have been gradually effaced by beds of sand and ashes <i>c, +d, e,</i> the surface at <i>e</i> being quite level. It will be +seen that, although the materials of the first layers have +accommodated themselves in a great degree to the shape of the +ground A B, yet each bed is thickest at the bottom. At first a +great many particles would be carried by their own gravity down the +steep sides of A and B, and others would afterwards be blown by the +wind as they fell off the ridges, and would settle in the hollow, +which would thus become more and more effaced as the strata +accumulated from <i>c</i> to <i>e.</i> Now, water in motion can +exert this levelling power on similar materials more easily than +air, for almost all stones lose in water more than a third of the +weight which they have in air, the specific gravity of rocks being +in general as 2½ when compared to that of water, which is +estimated at 1. But the buoyancy of sand or mud would be still +greater in the sea, as the density of salt-water exceeds that of +fresh.</p> + +<center><img src="../images/fig2.jpg" width="336" height="100" alt= +"Fig. 2. Section of strata of sandtone, grit, and congolmerate."> +</center> + +<p>Yet, however uniform and horizontal may be the surface of new +deposits in general, there are still many disturbing causes, such +as eddies in the water, and currents moving first in one and then +in another direction, which frequently cause</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 42">[ 42 ]</a></p> + +<br> + + +<p>irregularities. We may sometimes follow a bed of limestone, +shale, or sandstone, for a distance of many hundred yards +continuously; but we generally find at length that each individual +stratum thins out, and allows the beds which were previously above +and below it to meet. If the materials are coarse, as in grits and +conglomerates, the same beds can rarely be traced many yards +without varying in size, and often coming to an end abruptly. (See +Fig. 2.)</p> + +<center><img src="../images/fig3.jpg" width="307" height="307" alt= +"Fig. 3: Section of sand at Sandy Hill, near Biggleswade, Bedfordshire."> +</center> + +<p><b>Diagonal or Cross +Stratification.</b>—There is also another phenomenon +of frequent occurrence. We find a series of larger strata, each of +which is composed of a number of minor layers placed obliquely to +the general planes of stratification. To this diagonal arrangement +the name of “false or cross bedding” has been given. Thus in the +section (Fig. 3) we see seven or eight large beds of loose sand, +yellow and brown, and the lines <i>a, b, c</i> mark some of the +principal planes of stratification, which are nearly horizontal. +But the greater part of the subordinate laminæ do not conform +to these planes, but have often a steep slope, the inclination +being sometimes towards opposite points of the compass. When the +sand is loose and incoherent, as in the case here represented, the +deviation from parallelism of the slanting laminæ can not +possibly be accounted for by any rearrangement of the particles +acquired during the consolidation of the rock. In what manner, +then, can such irregularities be</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 43">[ 43 ]</a></p> + +<br> + + +<p>due to original deposition? We must suppose that at the bottom +of the sea, as well as in the beds of rivers, the motions of waves, +currents, and eddies often cause mud, sand, and gravel to be thrown +down in heaps on particular spots, instead of being spread out +uniformly over a wide area. Sometimes, when banks are thus formed, +currents may cut passages through them, just as a river forms its +bed.</p> + +<center><img src="../images/fig4.jpg" width="338" height="114" alt= +"Fig. 4"><img src="../images/fig5.jpg" width="329" height="93" alt= +"Fig. 5"></center> + +<p>Suppose the bank A (Fig. 4) to be thus formed with a steep +sloping side, and, the water being in a tranquil state, the layer +of sediment No. 1 is thrown down upon it, conforming nearly to its +surface. Afterwards the other layers, 2, 3, 4, may be deposited in +succession, so that the bank B C D is formed. If the current then +increases in velocity, it may cut away the upper portion of this +mass down to the dotted line e, and deposit the materials thus +removed farther on, so as to form the layers 5, 6, 7, 8. We have +now the bank B, C, D, E (Fig. 5), of which the surface is almost +level, and on which the nearly horizontal layers, 9, 10, 11, may +then accumulate. It was shown in Fig. 3 that the diagonal layers of +successive strata may sometimes have an opposite slope. This is +well seen in some cliffs of loose sand on the Suffolk coast. A +portion of one of these is represented in Fig. 6, where the layers, +of which there are about six in the thickness of an inch, are +composed of quartzose grains. This arrangement may have been due to +the altered direction of the tides and currents in the same +place.</p> + +<center><img src="../images/fig6.jpg" width="213" height="129" alt= +"Fig. 6: Cliff between Mismer and Dunwich."></center> + +<p> </p> + +<hr> +<p class="page"><a name="page 44">[ 44 ]</a></p> + +<br> + + +<center><img src="../images/fig7.jpg" width="367" height="206" alt= +"Fig. 7: Section from Monte Calvo to the sea by the valley of the Magnan, near Nice."> +</center> + +<p>The description above given of the slanting position of the +minor layers constituting a single stratum is in certain cases +applicable on a much grander scale to masses several hundred feet +thick, and many miles in extent. A fine example may be seen at the +base of the Maritime Alps near Nice. The mountains here terminate +abruptly in the sea, so that a depth of one hundred fathoms is +often found within a stone’s throw of the beach, and sometimes a +depth of 3000 feet within half a mile. But at certain points, +strata of sand, marl, or conglomerate intervene between the shore +and the mountains, as in the section (Fig. 7), where a vast +succession of slanting beds of gravel and sand may be traced from +the sea to Monte Calvo, a distance of no less than nine miles in a +straight line. The dip of these beds is remarkably uniform, being +always southward or towards the Mediterranean, at an angle of about +25°. They are exposed to view in nearly vertical precipices, +varying from 200 to 600 feet in height, which bound the valley +through which the river Magnan flows. Although, in a general view, +the strata appear to be parallel and uniform, they are nevertheless +found, when examined closely, to be wedge-shaped, and to thin out +when followed for a few hundred feet or yards, so that we may +suppose them to have been thrown down originally upon the side of a +steep bank where a river or Alpine torrent discharged itself into a +deep and tranquil sea, and formed a delta, which advanced gradually +from the base of Monte Calvo to a distance of nine miles from the +original shore. If subsequently this part of the Alps and bed of +the sea were raised 700 feet, the delta may have emerged, a deep +channel may then have been cut through it by the river, and the +coast may at the same time have acquired its present +configuration.</p> + +<p>It is well known that the torrents and streams which now</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 45">[ 45 ]</a></p> + +<br> + + +<p>descend from the Alpine declivities to the shore, bring down +annually, when the snow melts, vast quantities of shingle and sand, +and then, as they subside, fine mud, while in summer they are +nearly or entirely dry; so that it may be safely assumed that +deposits like those of the valley of the Magnan, consisting of +coarse gravel alternating with fine sediment, are still in progress +at many points, as, for instance, at the mouth of the Var. They +must advance upon the Mediterranean in the form of great shoals +terminating in a steep talus; such being the original mode of +accumulation of all coarse materials conveyed into deep water, +especially where they are composed in great part of pebbles, which +can not be transported to indefinite distances by currents of +moderate velocity. By inattention to facts and inferences of this +kind, a very exaggerated estimate has sometimes been made of the +supposed depth of the ancient ocean. There can be no doubt, for +example, that the strata <i>a</i>, Fig. 7, or those nearest to +Monte Calvo, are older than those indicated by <i>b</i>, and these +again were formed before <i>c</i>; but the vertical depth of gravel +and sand in any one place can not be proved to amount even to 1000 +feet, although it may perhaps be much greater, yet probably never +exceeding at any point 3000 or 4000 feet. But were we to assume +that all the strata were once horizontal, and that their present +dip or inclination was due to subsequent movements, we should then +be forced to conclude that a sea several miles deep had been filled +up with alternate layers of mud and pebbles thrown down one upon +another.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 46">[ 46 ]</a></p> + +<br> +<br> + <br> + + +<p>In the locality now under consideration, situated a few miles to +the west of Nice, there are many geological data, the details of +which can not be given in this place, all leading to the opinion +that, when the deposit of the Magnan was formed, the shape and +outline of the Alpine declivities and the shore greatly resembled +what we now behold at many points in the neighbourhood. That the +beds <i>a, b, c, d</i> are of comparatively modern date is proved +by this fact, that in seams of loamy marl intervening between the +pebbly beds are fossil shells, half of which belong to species now +living in the Mediterranean.</p> + +<center><img src="../images/fig8.jpg" width="295" height="261" alt= +"Fig. 8: Slab of ripple-marked (New Red) sandstone from Cheshire."> +</center> + +<p><b>Ripple-mark.</b>—The +ripple-mark, so common on the surface of sandstones of all ages +(see Fig. 8), and which is so often seen on the sea-shore at low +tide, seems to originate in the drifting of materials along the +bottom of the water, in a manner very similar to that which may +explain the inclined layers above described. This ripple is not +entirely confined to the beach between high and low water mark, but +is also produced on sands which are constantly covered by water. +Similar undulating ridges and furrows may also be sometimes seen on +the surface of drift snow and blown sand.</p> + +<p>The ripple-mark is usually an indication of a sea-beach, or of +water from six to ten feet deep, for the agitation caused by waves +even during storms extends to a very slight depth. To this rule, +however, there are some exceptions, and recent ripple-marks have +been observed at the depth of 60 or 70 feet. It has also been +ascertained that currents or large bodies of water in motion may +disturb mud and sand at the depth of 300 or even 450 feet.* Beach +ripple, however, may usually be distinguished from current ripple +by frequent changes in its direction. In a slab of sandstone, not +more than an inch thick, the furrows or ridges of an ancient ripple +may often be seen in several successive laminæ to run towards +different points of the compass.</p> + +<p class="fnote">* Darwin, Volcanic Islands, p. 134.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch1.html"> +Chapter I</a> / <a href="ch3.html">Chapter III</a></small> +</body> +</html> + |