« ՆախորդըՇարունակել »
of 1680 within a sixth of its diameter, or as near as 147,000 miles, its tail being of a like length.
There is one comet, however, that we seem to be somewhat better ac. quainted with than with this that paid us so near a visit, or indeed than with any other, from its having approached us visibly sor four times in succession, if not oftener. It was towards the beginning of last century that Mr. Halley was struck with the remark, that the general elements and character of the comets observed in 1531, 1607, and 1682, were nearly the same ; whence he concluded that the whole formed but one identical body, that took about seventy-six years to complete its eccentric orbit; and hence, although in consequence of this eccentricity, and its travelling amid a range of heavenly bodies that are altogether invisible to us, and whose influence seems to bid defiance to calculation, it is difficult to form an estimate of its progress, he ventured to suggest, that it would appear again, making due allowances for these incidents, lowards the close of 1758, or the commencement of 1759 : and he had the high satisfaction of seeing his prediction verified; the comet passing its perihelion March 12th, 1759, within the limits of the errors of which he thought his results susceptible. It is apparently this comet, which at this last period only excited the curiosity of astronomers and mathematic cians, that in 1456, or four revolutions earlier, towards the close of what are called the dark ages, spread such consternation over all Europe, already, indeed, terrified by the rapid successes of the Turkish arms, that Pope Cal. lixtus was induced to compose a prayer for the whole western church, in which both the Turks and the comet were included in one sweeping anathema.
Admitting the truth of Dr. Herschel's hypothesis, as we are now contemplating it, it is possible that some of the lately discovered planets, which are now attendant upon the sun, were formerly comets, whose orbits have for ages been growing progressively more regular, as well as their constitutional rudiments more dense; and such, indeed, is the opinion of M. Voigt, and of various other philosophers on the continent.
The object of the present and the preceding lecture has been to submit a sketch of the most obvious properties belonging to MATTER, so as to enable you to obtain a bird's-eye view of the general phenomena it is capable of assuming, and the general changes it is necessarily sustaining. From the qualities I have placed before you, of passivity, cohesibility, divisibility, and at. tractions of various kinds, must necessariiy result, according to the intensity with which they are called into action, the phenomena of liquidity, viscidity, toughness, elasticity, symmetry of arrangement, solidity, strength, and resilience. But the powers which thus perpetually build up the inorganic world, and to this our survey has been entirely confined, perpetually also destroy it: for the whole, as I have had occasion to observe, is a continued circle of action ; a circle most wise, most harmonious, most benevolent: and hence as one compound substance decays, another springs up in its place, and can only spring up in consequence of such decay.
There is, however, another lesson, if I mistake not, which we may readily learn from these lectures, however imperfectly delivered, and which is alto. gether of a moral character: I mean that of humility, in regard to our own opinions and attainments; and of complacency, in regard to those of others. After a revolution of six thousand years, during the whole of which period of time the restless ingenuity of man has been incessantly hunting in pursuit of knowledge, what is there in physical philosophy that is thoroughly and per fectly known even at the present moment ? and of the little that is thus known, what is there which has been acquired without the clash of controversy and the warsare of opposing speculations ? Truth, indeed,--for ever praised be the great Source of Truth, for so eternal and immutable a decree has at all times issued, and at all times will issue, from the conflict; but while we behold philosophers of the highest reputation, philosophers equally balanced in the endowment of native genius, proved by the great teacher Time to have been alternately mistaken upon points to which they had hon
estly directed the whole acumen of their intellect, how absurd, how contemptible is the fond confidence of common life! Yet what, indeed, when fairly estimated by the survey that has now been briefly taken of the sensible universe,-what is the aggregate opinion, or the aggregate importance of the whole human race! We call ourselves lords of the visible creation : nor ought we at any time, with affected abjection, to degrade or despise the high gift of a rational and immortal existence.-Yet, what is the visible creation ? by whom peopled ? and where are its entrances and outgoings? Turn wherever we will, we are equally confounded and overpowered: the little and the great are alike beyond our comprehension. If we take the microscope, it unfolds to us, as I observed in our last lecture, living beings, probably endowed with as complex and perfect a structure as the whale or the elephant, so minute that a million of millions of them do not occupy a bulk larger than'a common grain of sand. If we exchange the microscope for the telescope, we behold man himself reduced to a comparative scale of almost infinitely smaller dimension, fixed to a minute planet that is scarcely perceptible throughout the vast extent of the solar system ; while this system itself forms but an insensible point in the multitudinous marshallings of groups of worlds upon groups of worlds, above, below, and on every side of us, that spread through all the immensity of space, and in sublime, though silent harmony declare the glory of God, and show forth his handy work.
There are some subjects on which the philosopher is obliged to exercise nearly as much imagination as the poet; for it is the only faculty by which he can expatiate upon them. Such is a great part of the magnificent study upon which we have touched in our preceding lectures. Space, inmensity, infinity, pure incorporeal intelligence, matter created out of nothing, innumerable systems of worlds, and innumerable orders of beings;-where is the mind strong enough to grapple with such ideas as these? They at once entice and overwhelm us. “Reason copes with them till she is exhausted, and then gives us over to conjecture. Hence, as we have already seen, invention at times takes the place of induction, and the man of wisdom has his dream as well as the man of fancy.
Let us descend from such magnificent flights: let us quit the possible for the actual; and equally incapable of following up the fugitive material of which the visible universe consists, into its elementary principles and collective mass, let us examine it as far as we are able, in the general laws, structure, and phenomena it exhibits in the solid substance of the globe on which we tread.
It is this inquiry that constitutes the science of GEOLOGY, a brief outline of which is intended as a study for the present lecture ;-a science than which few are of more importance, but which is only at present in its infancy, and of course almost entirely indebted for its existence to the unwearied assiduity and discoveries of modern times.
The direct object of geology is, to unfold the solid substance of the earth to discover by what causes its several parts have been either arranged or disorganized—and from what operations have originated the general stratifi. cation of its materials, the inequalities of its surface, and the vast variety of bodies that enter into its make.
In pursuing this investigation, many difficulties occur to us. The bare surface, or mere crust of the earth's structure, is the whole we are capable of boring into, or of acquiring a knowledge of, even by the deepest clefts of vol. canoes, or the deepest bottoms of different seas. It is not often, however, that we have the power of examining either seas or volcanoes so low as to their bottom. The inhabitable part of the globe bears but a small proportion
to the uninhabitable, and the civilized an almost infinitely smaller proportion still. Hence our experience must be extremely limited; a thousand facts may be readily conceived to be unfolded that we are incapable of accounting for; and, at the same time, a variety of contradictory hypotheses to be formed with a view of accounting for them.
So far as the superficies of the earth has been laid open to us by ravines, rivers, mines, earthquakes, and other causes, we find it composed of a multi. tude of stony masses, sometimes simple, or consisting of a single mineral substance, as limestone, serpentine, or quartz; but more frequently compound, or constructed of two or more simple materials variously intermixed and united; as granite, which is a composition of quartz, felspar, and mica; and sienite, which is a composition of félspar and hornblend. These stony masses or rocks are numerous, and they appear to be laid one over the other, so that a rock of one kind of stone is covered by a rock of another kind, and this second by a third kind, and so on, in many instances, for a very considerable number of times in succession. In this superposition of rocks it is easily observable that their situation is not arbitrary. Every stratum occupies a determinate place; so that they follow each other in regular order froin the deepest part of the earth's crust, which has been examined, to the very surface. Thus there are two things respecting rocks which claim our peculiar attention-their composition and their relative situation. And independently of the rocks thus considered as constituting almost the whole of the earth's crust, there are other masses of fossil materials that must be likewise minutely studied; which traverse rocks in a different direction, and are known by the name of veins; as if the rocks had been split asunder in different places from top to bottom, and the chasms had been afterward filled up with the matter which constitutes the vein. And hence the veins which intersect rocks are as much entitled to our attention as the STRUCTURE and SITUATION of the rocks themselves.
Rocks, as to their STRUCTURE, may be contemplated under two divisions, simple and compound.
The simple division is, however, rather a speculative than a practical contemplation. It is possible that rocks, and of immense magnitude, may exist in parts of the globe we are not acquainted with, that are perfectly simple and unmixed in their structure; but it is seldom, perhaps never, that they have been actually found in such a state, at least to any considerable extent.
It is only under a compound form, therefore, or as composed of more than one mineral substance, that rocks are to be contemplated in our present survey of the subject; and in this form we meet with them of two kinds: CEMENTED, or composed of graips, or nodules, agglutinated by a cement, as sandstone and breccia or pudding-stone; and AGGREGATED, or composed of parts connected without a cement, as granite and gneiss. The component parts of the cemented rocks are often very multifarious; those of granite and gneiss much less so, consisting chiefly of felspar, mica, and quartz, with garnets, shorl, or hornblend occasionally intermixed with the mass. nite that forms the flag-stones of Westminster Bridge are supposed to have been brought from Dartmoor; and, like the rest of the Dartmoor granite, is remarkable for the length of its crystals of felspar, which in some instances are not less than four inches.
The aggregate rocks, like the cemented, are sometimes found of an indeterminate, but more generally of a determinate or regular form; and it is the office of that branch of mineralogy to which M. Werner has given the name of oryctognosy, to distinguish and describe them by these peculiarities. This is a branch into which I cannot plunge, for it would lead us from that general view of the science to which our present course of study is directed, into a detailed analysis. Those who are desirous of pursuing it in this line of developement may consult with great advantage Professor Jameson's System of Mineralogy, or M. Brogniart's Traité Elémentaire, or M. Cuvier's Essay on the Theory of the Earth, prefixed to his Fossil Remains. I can only observe, at present, that the total number of rooky masses, or different kinds of
rocks, whether simple or compound, which have been hitherto observed, amount to about sixiy; of which the principal seem to be the eight following: granite, gneiss, hornblend, limestone, wacke, basalt, quartz, and clay.
Let us next pass on, then, to consider their RELATIVE SITUATION. Of the different rocks thus glanced at, and placed over each other, the whole crust of the earth is composed, to the greatest depth that the industry of man has been able to penetrate; and I have already observed, that with respect to each other, they occupy a determinate situation, which holds invariably in every part of the globe. Thus, limestone, excepting under particular circumstances, hereafter to be explained, is nowhere found under granite, but always above it. This general view of the subject may, indeed, induce a supposition that every separate layer which constitutes a part of the earth's surface is extended round the entire globe, and wrapped about the central nucleus, like the coats of an onion; the kind of rock that is always lowest, or nearest the centre, uniformly supporting a second kind, and this second kind a third, and so on. Now, though the different kinds or layers of rocks do not in reality extend round the earth in this uninterrupted inanner-though, partly from the inequality of the nucleus on which they rest, partly from their own inequality of thickness in different places, and partly from other causes, the continuity is often interrupted-yet still we trace enough of it to con. vince us that the rocks which constitute the crust of the earth, when con templated upon a large scale, are every where the same, and that they inva. riably occupy a like situation with respect to each other.
The labours of Mr. Kirwan and M. de Saussure gave the earliest hints upon this subject; and the geological theories of Prosessor Werner of Freyburg, and of M. de Cuvier of Paris, are entirely founded on the same. These theories, though derived in some measure from different sources of mineralogical study, coincide not merely in their general outline, but in all their more prominent parts, and only differ in their mode of accounting for the more limited or local deposites.
M. Werner, “ from whom alone,” to adopt the language of M. de Cuvier, “we can date the commencement of real geology,” so far as respects the mineral natures of the strata, divided in his first view of the subject, all the various rocks that enter into the solid crust of the earth, into five classes.
Of these the First Class consists of those rocks which, if we were to suppose each layer to be extended over the whole earth, would lie lowest, or nearest the centre, and be covered by all the rest; it comprises seven distinct sets, as granite, gneiss, mica-slate, clay-slate, a peculiar kind of porphyry, sienite, and a peculiar kind of serpentine. Of these granite lies the undermost, and sienite the uppermost; and in the midst of several of them we meet with beds of 'not less than eight other kinds of rock, as though dropped into them by accident—as topaz, another kind of porphyry, serpentine, limestone, flini-slate, and trap, quartz, and gypsum; which are hence called subordinate rocks of this class, and which extend the whole number of sets belonging to it to fifteen.
These are supposed to have been earliest produced, and when the earth first emerged from a state of chaos to a state of order; and are hence denominated PRIMITIVE FORMATIONS. They are distinguished by the following character. Not a single relic of either animal or vegetable petrifaction is to be found in any of them. The lowermost or older contain no carbonaceous matter; which is discoverable but very sparingly in the superior or newer. They are all chemical combinations, and generally crystallized; the crystallized appearance being most perfect in the oldest, and gradually becoming less perfect in the newer formations. I have already observed that the whole of this scale of formations does not regularly coat the nucleus of the earth; so little so, indeed, that sometimes even the granite itself, the lowermost rock of all, is left bare, and not pressed down or coated by a deposite of any other kind of rock: and so of the rest. Wherever this deficiency takes place, the rock thus left at liberty rises uniformly higher than it is found to do where pressed upon and invested with its common coatings. But every rock does not, under such circumstances, rise equally high, or with an equal degree of freedom; for granite rises highest of all; and hence we frequently find it composing the tops of our loftiest chains of mountains, as well as the basis of the earth's solid crust. It forms the great body of the Swiss mountains and the Alps, though gneiss is here also found in great abundance.
The level of gneiss, when left at equal liberty, is a little lower than that of granite. It constitutes the vast mass of the Carpathian mountains, that divide Transylvania and Hungary from Poland.
The level of mica-slate is lower than that of gneiss, and the level of clayslate lowest of all. So that there is a regular sinking of these respective levels from granite to clay-slate : while the newer porphyry and sienite are often laid over their summits, as though these two formations had been deposited long after the production of the others; an idea which is still farther strengthened by our meeting occasionally with a bed of breccia, or puddingstone, composed of fragments of the older or lower rocks, capping the gneiss, granite, or other formation before the porphyry or sienite has been deposited.
The SECOND CLASS of rocks, or that which, when the number of coatings is complete, lies immediately over the preceding, consists of gray-wacke slate, and a peculiar kind of limestone, greenstone, and amygdaloid ; together with subordinate masses of the proper primitive formations, sienite, porphyry, and granite; as though some portions of these had become crystallized after the rest, along with the next layers in succession, or had been separated from the parent rocks by some early commotion. Gray-wacke, which is a concrete term, denoting a conglomerate rock of a peculiar kind, having a basis of clayslate, and being studded or otherwise intersected with portions of quartz, felspar, and scales of mica, may be exemplified by what in Cornwall is called killas, a far more euphonous word; and hence gray-wacke and gray-wacke slate may be distinguished by the terms amorphose and schistose killas. The Cornish killas lies directly over the granite of that county, which possesses the character ascribed by Werner to granite of the highesi antiquity.*
These formations, for the most part, irregularly alternate with each other, instead of preserving one regular and successive order, as the different sets of the primitive formations do; excepting that the limestone appears usually undermost, and placed, as the basis of the rest, upon the sienite or uppermost of the first class. It is in this second class of formations that petrisactions first make their appearance; and it deserves particular attention that they are uniformly confined, both in the animal and vegetable kingdoms, to those of the lowest links in the scale of organization; and even among these to species which are at present altogether unknown, and which appear therefore to be totally extinct. Thus the animal petrifactions consist entirely of ammonites, mytilites, unknown corals, and other zoophytic worms; and the vegetable petrifactions of reeds, ferns, and other palin-like plants, mosses, and other cryptogamic productions, which occupy the lowest part in the scale of vegetable life, as zoophytic worms do among animals. It is here, also, that carbonaceous matter, which is chiefly of vegetable origin, first makes its appearance in any considerable quantity.
To this class of rocks, therefore, M. Werner has given the name of TRANSITION FORMATIONS; as believing them to have been produced while the earth was in a state of transition from inorganic matter to organic life,-from an uninhabited to an inhabited condition. The date of their formation, however, is proved even from their natural appearance, to have been very remote; since, as already observed, the whole of the petrifactions which they contain consist of plants and animals, not only of the very lowest species, but which now seem to be altogether extinct.
The THIRD CLAss of rocks is denominated FLOETZ, that is, Flat Or HORIZONTAL FORMATIONS, in consequence of their usually appearing in beds much more nearly horizontal than the preceding. They lie immediately over the transition-class, and consist of the twelve following distinct sets of rock, each
* See Allan's remarks on the transition-rocks of Werrer, in Thomson's Annals of Philos. vol. iii. p. 23 Compare with Janeson's dofinition of the same. Id. Feb. 1817, p. 17.