PHILLIPS'S GEOLOGY,
WORKS on Geology appear to us generally to fail in the mutter of history and of proof. They not only plunge in mediae res, but, unlike the poet, they never tell us what has occurred before their Leta, in a rapid, connected, and satisfying manner; their ut- most performance in this direction merely stretching to a meagre notice of the theories which some former geologists have maintained. They are still more deficient in the matter of proof, or at least their deficiency is more felt. Instead of a masterly exposition of the first principles of the science, of the evidence upon which it rests, and of the way in which the truth of the testimony was established, the reader meets with a variety of laws or general facts, which to him are merely assertions : and, in learned but un- skilful hands, these things are stated in uncouth terms, which are also unintelligible unless he is acquainted with chemistry, mine- ralogy, and other auxiliary sciences. Hence, though the wonders of geology elevate, enlarge, and astonish the mind, geological ex- positions are neither satisfy ing nor popular. In despite of a very orderly arrangement, and a minute subdi- vision of his subject, this defect is a leading characteristic. of Mr. PHILLIPS'S Treatise on Geology. It is more adapted to the somewhat advanced student, who wishes a methodized coup (reef', than to the tyro, who has every thing to learn. The arguments, so far as they go, are conclusive; but they begin in the.middle, or stop short; and the more imposing and larger views are not reached by the author in this first volume, its subjects chiefly embracing the more technical parts of the science. It is likely, however, that when Mr. PHILLIPS comes to speak of the operations of natural agents upon the existing surface of the globe, he may rise with his theme; as he does in this description of the
EFFECTS OF RIVERS.
A river thus fed by springs of water not pure, partially filled with earthy matter, flowing with various velocities through soil and among rocks of un- equal resisting power, and formed of particles of different magnitude and specific gravity, must exhibit in its long course a great diversity of appearance.. Some rocks and soils it may corrode chemically, others it may grind away by its own force and the aid of the sand and particles which go with it : from steep slopes it must, in general, transport away all the loose materials ; but when its course relents, these must drop and augment the land. The finest particles are first taken up and last laid down, the larger masses make the shortest transit. Rivers on whose course no lake interposes its tranquillizing waters, may be considered as constantly gathering, incessantly transporting, and continuilly depositing earthy materials. It is of course principally in times of flood that they both gather the most materials, and transport them furthest; yet even in the driest season, the feeblest river does act on its bed, wears by little awl little even the hardest stones, and works its channel deeper or wider. This it does, partly by the help of some chemical power, from carbonic acid and other admixtures, but principally by the grinding agency of the sand, pebhles, which it moves along. In times of flood, these act with violence lake so many hammers on the rocks, ploughing long channels on their surface, or whirling round and round in deep pits, especially beneath a fall, or where the current breaks into eddies over an uneven flour of stone. This is admirably seen at Stenkrith Bridge, in Westmoreland, under the waterfalls about Blair Athol, and in North Wales, and indeed very commonly. Not uufrequently on mountain sides or tops, far from any stream or;chaunel, phenomena somewhat similar occur, sometimes the effect of rain, sometimes, we may suppose, the remaining evidence of the former passage of running water, when the levels of the country were differently adjusted. As the slopes arc greatest in the upper parts of vallies (generally), and gra- dually flatten towards the sea, it is commonly observed that, from all the upper parts of these vallies, rivers abstract large quantities of the finer matter, and in times of inundation, not a little of the coarser fragments of rocks: much of this is deposited in the lower ground, where the current is more tranquil, and generally (unless the river be very deep) slower. We must indeed suppose, that everywhere some wearing effect on its bed and sides is produced by every river, even to its mouth ; but this effect grows almost insensible far from the
high ground which gives birth to the streams ; and long ere we approach the
estuary, the wide, flat meadows, which fill the whole breadth of the valley for miles in length, show what a mass of materials has been drifted away from the higher ground. Finally, where the tides and fleshes meet, the sediment of both is disposed to drop ; and some rivers may be viewed as sending little or no sediment to the sea.
Thus the whole effect of drainage, including all the preliminary influences of the atmosphere, rain, springs, &c. is to waste the high ground and to raise the
low—to smooth the original ruggedness of the valley in which it flows, by removing prominences and filling up hollows : and notwithstanding the length of years that rivers have flawed, they have, in general, not yet completed this work ; they still continue to add materials to the lower ground, and, in a few instances, to carry out sediment into the sea.
The whole surface of the earth, then, is changing its level, by the mere preci- pitations of the atmosphere, and their subsequent effects; the high land sinks, and the low land rises.
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