MACKIE — ROCKS AN^D THEIR TEACHINGS. 



97 



near. We perceive at once they are the dshris of ancient cliffs, and 

 our thoughts turn to the rough waves thundering at their base, 

 and foaming back from their incessant toil, repulsed by the crags and 

 rocks, for weeks and months, only to prevail over and hurl down the 

 stony masses in the end ; then, rolling and labouring the broken frag- 

 ments along the shore, day and night, with the same monotonous 

 cadence as soothes us now on the margins of the great waters, 

 to fashion them into smooth and polished stones. Twenty feet 

 of modern pebbles, a quarter of a mile broad, form a great beach. 

 The great beach of the old red sandstone was probably two thousand 

 feet thick. 



Perhaps, however, those wonderful accumulations of the Devonian 

 period ought to be compared with the time and water- worn, 

 semi-angular, and partially-rounded talus of the N^orth American 

 mountains, which is strewn on the borders of the great lakes ; the 

 boulders of such origin being usually larger and less smooth than the 

 pebbles of the sea-shore, which are subject to more rapid and far more 

 powerful attrition. The Permian conglomerates may also have 

 proceeded from a like source ; and the intermediate coal-mea- 

 sures, in their partly terrestrial and partly fresh -water and sestuarine 

 characters and conditions, seem in no way discouraging to the 

 comparison. 



We see, moreover, in the microscopic structure of many of those 

 finer siliceous rocks, which are composed of a segregation of the solid 

 cases of infusoria, and of limestones compounded of nearly equally minute 

 entromostraca and foraminifera, the slowly-formed life-elaborated pro- 

 duce of the tranquil lake, or of the profound abysses of the deep. 

 Our ponds and inland seas give us the key to the one, the soundings 

 for the great Atlantic telegraph that to the other. The silent language 

 of the past, in which these facts are written, appears at first strange 

 and unintelligible, as the tongue of our Saxon forefathers does to us 

 now. But as the languages of the ancient peoples are interpreted by 

 our own, and by those of contemporary nations, so the events and 

 changes of the past conditions of our planet can only be understood by 

 comparison with the phenomena going on around us. 



It is commonly stated that we are acquainted with something like a 

 thickness of ten miles of the earth's crust — - and what is that to the 

 eight thousand miles diameter ? But we do not know absolutely so 



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