mortal 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 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 handywork.

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LECTURE VI.

ON GEOLOGY.

THERE are some subjects on which the philosopher is obliged to exercise nearly as much imagination as the poet; for it is almost 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, immensity, 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 phænomena it exhibits in the solid substance of the globe on which we tread.

It is this enquiry 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 disorganised—and from what operations have originated the general stratification 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 volcanoes, or the lowest 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 civilised 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 multitude 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 felspar 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 from 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 afterwards 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 specu

lative 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 grains, 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. The granite that forms the flagstones of Westminster Bridge is 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 which I cannot pursue, for it would lead us from that general view of the science to