LYELL'S ELEMENTS OF GEOLOGY.

IN this volume Mr. Lansss has successfully accomplished 7e7gla important ends. He has furnished the student with an outline of the science, perfectly intelligible in its principles, thoush requir- ing instruction—perhaps direct instruetion by a master—in its de- tails, so far as they relate to the chemical cotnpositi in of rocks. Giving the coup de grace to several received theories regarding the consecutive formation of different rocks, Mr. INELL leaves the origin and original matter of our world in medio ; showing that the geologist at present should confine himself toilets, and to such limited prineiples as are clearly deducible from them. By bringing the calmness of a philosophical mind to the exposition of the wonders of geology, he strips them of some of their more extravagant features: leading one to infer that the earlier systems of nature, however contrary to our no n, were regular and quiet— that the prinurval world was not, like the hell-broth of a witch's cauldron, a chaotic aid monstrous stale, where nature effected her purposes in turbulent commotion, but a regular theugh vast la- boratory, analogous to that which is now displayed to the living observer. As, judgmg from their language, these views are new even to those who write upon the subject, and certainly very con- trary to those which the public entertain, we shall *freely avail ourselves of Mr. LYELL'S pages to furnish a succinct view of his theory. or rather of the theory deducible from his Elements.

It was held, and may still be by many, that the materials of the world were originally in a state of lesion, with the moisture, sub- sequently to become seas and rivers, floating in a stale of mist round the fiery mass. As this gradually cooled, it formed granite ; and the mist condensing, descended on its surface, causing vast collections of hot water — " thermal oceans," in

which life could not exist. By successive steps, never very intelligibly accounted for so far as we have read, parts of this granite were transformed into various kinds of stone, marble, and slate,—the waters dining this process undergoing a decrease of temperature and a change of composition fitting them to aid in supporting life. All rocks of the kind just. enmnerated were called by geologists primary, as they conceived them to be the first formed, and in them no remains of animals or vegetables were found. Another series of rocks, containing a few fossils, vegetables, and atiimals, as high in the scale of creation as fishes, were called transition, and were held to have been formed from the primary rocks, by the combined action of the air, the water, and temperature. To this class succeeded the secondary and tertiary formations; containing in their various rocks, or popularly soils, none and more of animal remains more and more allied to those at present existing.

By a series of reasons, too technical to recapitulate and too scattered to collect, Mr. LYELL altogether repudiates this theory. Of the primitive matter of the world he conceives we have at present no data on which to form an opinion : of the relative age particular rocks in local strata we can draw a conclusion more or less certain according to the evidences which they furnish, but we cannot decide which class of rocks is primitive. We may fiequently be enabled to say this particular stratum of granite is older than this particular stratum of chalk, or this sandstone is older than this coal; but we cannot undertake to say that grani'e was formed before chalk, or chalk before granite, and so forth, for no evidence of this exists. There are strong grouuds for assert- ing, that large beds of chalk are now in the course of formation, as well as other classes of rocks arranged by former geologists under different series; whilst it is at least probable that granite and other primary rocks may now be forming. in the bowels of the earth. Neither dues Mr. LYELL seem to lean to the notion—most assuredly he does not lead his reader to entertain it—that the whole earth was at any time broken up by a scene of frightful violence, during which its then iubabitants were destroyed and engulfed. On the contrary, considerable changes are taking place around us, though very slowly,—the sea encroaching in some places and retiring in others ; rivers forming largs deltas of marshy hied; esteusive districts, as in Sweden and other coun- tries, being imperceptibly raised many feet above the level of the sea ; earthquakes, avalanches, and the immense masses of ice in the uninhabitable arctic regions show the forces that may be operating even now; whilst the enormous furnaces which feed existing volcanoes must effect great chemical changes in the interior of the earth, as their eruptions produce more limited effects upon its surface. Many of the fossil remains which are preserved, " themselves their monument," it is evident died a natural death, as millions of creatures are daily dying around us, and were floated down rivers to the deposits where they were slowly entombed. The creatures whose death and burial must have been simultaneous, perished, there is every reason to believe, in small numbers, by some local disturbance, of which we have examples in turbid floods, in eruptions of marine volcanoes, and in earthquakes. We are inclined to infer that few if any species or genera were suddenly extinguished; but gradually died away

by the earth becoming more unfit for their sustenance, and perhaps by the increase of their natural enemies ; just as we now find whole races of animals become extinct before the advance of man, or districts formerly populous are now from natural changes abandoned and desolate,--a termination, perhaps, more truly pain- ful and deplorable, though less frightful and revolting to the mind, than violent and abrupt extinction.

The reader must not conclude, because Mr. LYELL dissi- pates the melodramatic extravaganzas of geology, that no natural

marvels remain; or because he aids in overturning an untenable theory, that he reduces the science to a mere collection of facts. To the wonders we shall take an opportnnity of recurring, after we have given a brief outline of the author's principles.

According to Mr. LYELL, there are four classes of rocks ; (the word rock e'being "applied indifferently by geologists to all

strata, whether soft or stony ;') which may be classified accord- ing to their origin and their relative age. The subject of age involves too many abstruse scientific points to be discussed in our columns : the classification of rocks according to their origin is as follows.

I. Aqueous rocks; produced by watery action, and which cover a larger part of the earth's surface than any others.

2. Volcanic rocks; whose origin is beyond all question referable to fire, and which are for the most part unstratified and are de- void of fossils. Most of the volcanic rocks produce a fertile soil on disintegration.

3. Plutonic rocks; comprehending all the granites and certain porphyries, which are nearly allied in some of their characters to

volcanic formations. The rocks of this class are highly crystal- line and are unstratified and destitute of organic remains. As no such formations are now going on at the surface of the earth, no

positive proof can be produced as to their origin ; but from many reasons which Mr. LYELL adduces, " it has been inferred that the granites have been formed at great depths in the earth, and have cooled and crystallized slowly under enormous pressure where the contained gases could not expand."

4. Metamorphoric rocks include "gneiss, mica-schist, clay-slate, chlorite-schist, marble, and the like." They resemble the plutonic rocks, in containing no traces of organic remains ; they are often as crystalline as granite ; but, unlike the plutonic rocks, they are sti atitled. And Mr. LYELL argues that they have been aqueous rocks, subjectel to a beat from the plutonic formations sufficiently

powerful to change their composition and obliterate all traces of fossils they possessed, but not to destroy their stratiferous character. The inference is possible, but not conclusive, that granite itself, once held to be the product of the cooled material ere water was, is often merely aqueous rocks so fused as to ebliterate even their strata.

Referring the curious to the Elements themselves, for fuller particulars respecting the three last classes, we will collect together a few points relative to the present and past formation of aqueous rocks ; the present consisting of description or experiment, the past of reasoning from analogy ; and both furnishing a fair speci- men of Mr. LvaLL's elementary style.

AQUEOUS ROCKS

Are stratified or divided into distinct layers or strata. The word stratum means simply a bed, or any thing spread out or strewed over a given surface ; sod we infer that these strata have been generally spread out by the action of water, from what we daily see taking place near the mouths of rivers, or on the

land during temporary inundations. For whenever a running stream, charged with mud or sand, has its velocity checked, 89 when it enters a lake or sea, or overflows a plain, the sediment, previously held in suspension by the motion of the water, sinks by ite own gravity to the bottom. In this manner layers of mull and sand are thrown down one upon another.

If we drain a lake which has been fed by a small stream we frequently find at the bottom a series of deposits, disposed with considerable regularity, one above the other ; the uppermost, perhaps, may he a stratum of peat, next be- low n more dense and solid variety of the same material; still lower a bed of laminated shell-marl, alternating with peat or sand ; and then other beds of marl, divided by layers of clay. • * In the estuaries of large rivers, such as the Ganges and the Mississippi, we may observe, at low water, pheenomena analogous to those of the &elated lakes above-mentioned, but on a grander scale, and extending over areas several bun. dred miles in length and breadth. When tile periodical inundations subside, the river hollows out a channel to the depth of many yards through horizontal beds of clay and sand, the ends of which are seen exposed in perpendicular cliff.. These beds vary in colour, and are occasionally characterized by con- taining drift-wood or shells. The shells may belong to species peculiar to the river, but are sometimes those of mariue teetacea, washed iutu the mouth of the estuary during storms.

The annual floods of the Nile in Egypt are well known, and the fertile de. posit of mud which they leave on the plains. This mud is stratified ; the thin

layer thtown down in one season differing slightly in colour front that of a pie. whets year, and being separable from it, as Las been observed in excavations at .Cairo and other places. When beds of sand, clay, and marl, containing shells and vegetable matter, are found arranged in the same manner in the interior of the earth, we a-Tribe to them a similar origin ; and the more we examine their character,' its minute detail, the more exact do we find the resemblance. Thug, for example, at various heights and depths in the earth, and often far from seas, lakes, and xivers, we meet with layers of rounded pebbles composed of different rocks mingled together. They are like the pebbles formed in the beds of torrents and rivers, which are carried down into the sea wherever these descend from high grounds bordering a coast. There the gravel is spread out by the waves and currents of the ocean over a considerable space ; but during seasons of drought,

the torrents and rivers are nearly dry, and have only power to convey tine sand or mud into the sea. Hence, alternate layers of gravel and fine sediment mew emulate under water ; and such alternations are found by geologists in the interior of every continent.

If a stratified arrangement, and the rounded forms of pebbles, are alone suf- ficient to lead us to the conclusion that certain rocks originated under water,

tkis opinion is further coufirined by the distinct and independent evidence of fossils, so ahundsutly included in the earth's crust. By a fossil is meant any body, or the traces of the existence of any body, whether animal or vegetable, which has been.buried in the earth by natural crimes. Now the re ' animals, especially of aquatic species, are found almost everywhere iintl:d,e- in stratified rocks. Shells and corals are the most frequent ; and wimth

thea are often associated the bones and teeth of fish, f ts of leaves, and other organic suh Fossil silraeghmeleInsofoffowrinli'suimeliPreessiniat

abound in the sea are met with far inland, both near the surface and at7 depths below it, as far as the miner can penetrate. They occur at all heiehall above the level of the ocean, having been observed at an elevation of from 8O to 9,000 feet in the Alps and Pyrenees, more than 13,000 feet high is the Andes, and above 15;000 feet in the Himalayas.

THE DELUGIANS ANSWERED.

When geology was first cultivated, it was a general belief that these mari shells and other fossils were the effects and proofs of the general Deluge. 134eue all who have carefully investigated the phtenomena have long rejected this doe. trine. A transient flood might be supposed to leave behind it, here and there upon the surface, scattered keeps of mud, sand, and shingle, with shells coe. fusedly intermixed; but the strata containing fossils are not siiperficialdepseu, and do not cover the earth, but constitute the entire mass ofmdoouftutahirneo.cellat

th has been also the favourite notion of some modern writers, who are aware at fossil bodies cannot all be referred to the Deluge, that they and the strata ia which they are entombed may have been deposited in the be

during a period of several thousand years which intervened between the men tion of man and the Deluge. They imagine that the antediluvian bed of the ocean, after having been the receptacle of many stratified deposits, became core vetted, at the time of the Flood, into the lands which we inhabit; aria eau the ancient continents were at the same time submerged, and became the Ind of the present sea. This hypothesis, however preferable to the diluvial theory, a9 admitting that all foesiliferout strata were slowly and successively thrown down from water, is yet wholly in ulequate to explain the repeated revolutions which the earth has undergone, and the oigns which the existing continents ez. hibit, in most regions, of having emerged- from the ocean at an aera far more remote than four thousand years from the present time. It will also be seen ia the oequel, that many distinct vets of sedimentary strata, each several hundreds or thousands of feet thick, are piled one upon the other in the earth's crust, each containing their peculiar fossil animals and plants, which are distinguish. able, with few exceptions, from specks now living. The mams of some of these strata consist almost entirely of corals, others are made up of shells, lithers of plants turned into coal, while some are without fossils. In one set of strata, the species of fossils are marine; in another, placed immediately above or be. low, they as clearly prove that the deposit was formed in an estuary or lake. When the student has more fully examined into these appearances, he will be- come convinced that the time required for the origin of the actual continents must have been fir greater than that which is conceded by the theory above al- luded to, and that no one universal and sudden conversion of sea into land ea account for geological appearances.

Passing a clear, curious, and convincing series of proofs as to the slow formation of strata in aqueous rocks, we will next quote some of Nature's quiet wonders.

It has been already remarked that there are rocks in the interior of conth nents, at various depths in the earth and at great heights above the sea, almost entirely made up of the remains of zoophytes and testacea. Such messes may be compared to modern oyster-beds and coral reefs; and, like them, the rate of increase must have been extremely gradual. But there are a variety of stony deposits in the earth's crust now proved to have been derived from plants sod animals, of which the organic origin was not suspected until of late years, eves by nateralists. Great surprise was therefore created by the recent discovery of Professor Ehrenberg of Berlin, that a certain kind of siliceous stone, called tripoli, Was entirely composed of millions of the skeletons or cases of mine scopic animalcules. The substance alluded to has long been well known in the arts, being tried its the form of powder for polishing stones and metals. It has been procured, among other places, from Bain, iu Bohetnia, where a single stratum, extending over a wide area, is no less than fourteen feet thick. This stone, when examined with a powerful microscope, is found to consist of the siliceous cases of infusnria, united together without any visible cement. It is difficult to convey an idea of their extreme minuteness ; but Ehrenberg, esti. mates that in the Bilin tripoli there are 41,000 millions of individuals of the Gaillonella distans in every cubic inch, which weighs about ea.9 grains, or about 187 millions in a single grain. At every stroke, therefote, that we make with this polishing powder, several millions, perhaps tens of millions of perfect fossils, are crushed to atoms,

The length of time requisite to attain skill in a pursuit, and then to collect materials worthy of the exercise of both skill and genius, is a subject of something like regret with thoughtful minds; but if we look at the slow operations of Nature, the wonder is that an individual mortal can accomplish as much as is some- times done. See what countless years, how many millions of lives, are necessary in the formation of a chalkpit. Generations must must have lived and died to furnish Boniface with the bit of chalk for his score

It had been often suspected before that white chalk might be of animal origin, even where every trace of organic structure has vanished. This bold ides wu partly founded on the fact, that the chalk consisted of pure cal bonate of lime, such as would result from the decomposition of testaeea, echini, and corals, nod in the passage observable between these fossils when halt' decomposed into chalk. But tido conjecture seemed to malty naturalists quite vague and visionary, until its probability was strengthened by new evidence brought to light by modem geologists. We learn from Lieutenant Nelson, that, in the Bermuda Islands, there cas several basin, or lagoons almost surrounded and enclosed by reefs of coral. At the bottom of these lagoons, a soft white calcareous mud is formed by the de- composition of Eschara. Flustra, Cellepora, and other soft corallinee. This mud, when dried, is (indistinguishable from common white earthy chalk; and some portions of it, presented to the museum of the Geological Society of London, might, after full examination, he mistaken for ancient chalk, but idr. the labels attached to them. About the same time, Mr. C. Darwin observed similar facts in the coral islands of the Pacific ; arid canoe also to the opinion, that much id' the soft white mud found at the bottom of the sea near coral reek has passed through the bodies of worms, by which the stony masses of coral are everywhere bored ; and other portions through the intestines of fish; for certain gregarious tieh of the genus Spares are visible through the clear weir;

when we recollect how the fossilise was formerly puzzled by meeting with cer

grazing herds ort

browsing quietly, in great numbers, on living corals, like

graminivorotei quadrupeds. On opening their -bodies, Mr. Darwin found the intestines filled with impure chalk. This circumstance is the more in Pot tin bodies, called cones of the larch, in chalk, which were afterwards rent nized by Dr. Buckland to be the excrement of fish. These spiral caprolites,■1 s the wide* and bones of fossil fish in the chalk, are composed chiefly of ph,* phate of lime.

We will close our extracts with a passage that may &rd.! many—an account of miniature prinneval monsters yet existing

" Some bright little isles of their own, In a blue summer ocean far off and alone."

The author has been describing the gigantic creatures whose fos-

sils attest their existence.

po, the last twenty years, anatomists have agreed that these extinct saurians mot hue inhabited the sea, although no living marine reptile was known. Tbsy argued that, tm there are now CiteIonians, like the tortoise, living in fresh r, and others, as the turtle, frequenting the ocean, so there may have been 'it lc some saurians proper to salt, others to fresh water. The recent die. saver!, however, of a maritime saurian, has now rendered it unnecessary to weul te on such possibilities. This creature was found in the Galapagos 904 during the visit of H. M. S. Beagle to that archipelago in 1835; and iteliabits were then observed by Mr. Darwin. The islands alluded to are United under the equator, nearly six hundred miles to the westward of the mart of South America. They are volcanic, Nome of them being three shomand Or four thouund feet high ; and one of them, Albemarle Island, seresty.five miles long. The climate is mild, very little rain falls • and, in the whole archipelago, there is only one rill of fresh water that reaches the coast.

The soil is for the most part dry and harsh, and the vegetation scanty. The and insects, are, with very few exceptions, of species birds, reptiles, plants,

bled owhere else in the worlol, although all partake in their general form of on American character. Of the mammalia, says Mr. Darwin, one species Awe appears to be indigenous, namely, a large and peculiar kind of mouse; but the number of lizards, tortoises, and snakes is so great, that it may be called a bad of reptiles. The variety, indeed, of species is small; but the individuals of each are in wonderful abundance. There is a turtle, a large tortoise, (Ta- rok Indicus,) four lizards, and about the same number of snakes, but no kegs or toads. Two of the lizards belong to the family Ismarsidm of Bell. and to a peculiar genus (Anablyrhynchus) established by that naturalist ; and to named from their obtusely truncated head and short snout. Of these lends, one is terrestrial in its habits, and burrows in the ground, swarming everywhere on the land ; having a round tail, and a mouth soinevrhat resembling inform that of the tortoise. The other is aquatic, and has its tail flattened laterally, for swimming. " This marine saurian, " says Mr. Darwin, "is ex- tremely common on all the islands throughout the Archipelago. It lives ex- clusively on the rrcky sea-beaches, and I never saw one even ten yards in-shore. The usual length is about a yard, but there are some even four feet long. It is ifs dirty hack colour; sluggish in its movements on the land, but, when in the water, it swims with perfect ease nod quicknese, by • serpentine movement of its body and flattened tail, the legs during this time being motionless, and

elowlv collapsed on its sides. Their limbs and strong claws are admirably skied for crawling over the rugged and fissured masses of lava which every.

ulore form the coast. In such situations, a group of six or seven of these hideous reptile may oftentimes be seen on the black rocks, a few feet above the turf, basking in the sun with outstretched legs."