Aspects of the Earth : a Popular Account of Boone
Familiar Geological Phenomena. By N. S. Shaler, Professor of Geology in Harvard University. Illustrated. (Smith, Elder, and Co.)—This excellently illustrated volume may as confidently be recommended to the scientific as to the general reader. It principal merit lies in the lucid and interesting manner in which the globe and its surroundings are shown to be by no means in an inert con- dition, but the resultant of ceaseless changes, massive and molecular. Absolute rest, indeed, is probably unknown to the cosmos, and the sum of things is the sum of motions, vibrational and translational. So far as our world is concerned, the sun's heat and the inner heat of our earth are the main ultimate causes of all the motional changes which have shaped and modified, and still shape and modify, the surface and interior of the globe, and the movements and distributions of its envelope. It is the sun's heat which lifts the watery vapours from the seas, and gives birth to the air-currents which carry the clouds over the land to fall in rain that, gathering into streams and rivers, sculptures out the valleys and the hills, bears down stones, sand, and sediment to form geological strata, or, solidified into massive ice-coverings, grinds the earth's surface into drift and mud. In this way the land is being gradually washed down into the sea, and were the process unhindered, the earth in time, as Alfred de Musset says,— " ras6, sans barbe iii eheveux,
Oomme on grand potiron roulerait thins les deux."
but covered with an even depth of ocean. Fortunately, however, for terrestrial life, the action of the earth's own heat is antagonistic to that of the sun's heat. In its passage from the centre, where it is measurable only in tens of thousands of degrees, to the exterior of the globe, the earth's heat traverses strata within some thirty miles of the surface which still retain more or less of the water of deposition, and this occluded water, in the state of intensely super-heated steam, distending the vast masses of rock with irresistible force, causes, together with secular contraction, the various motions of the solid crust, slow or sudden, which lift up continents, or tracts of land, or coast-lines, or detached islands, at such a rate that the struggle between denudating and elevating agencies results in the partial victory of the land, realised in the configuration of land and ocean as we see it; but whether or not the victory be of a permanent character, no one can yet say. Several most interesting chapters of the
book deal in detail with the origin and life-history, so to speak, of rivers and river-valleys, winds and storms, and will be easily
understood and appreciated by the least scientific reader, though possibly he will find those treating of the stability, or rather the instability, of terra firma, and of the causes and effects of volcanic action, more dramatic, and so more attractive.
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