CAMBRIDGE ESJsAYS : HOPKINS^ ^^GEOLOGV. 



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dominence of this character. There are some beds, however, which 

 consist principally of rounded pebbles, or other masses of various sizes, 

 which must have had their present farms when first arranged in the 

 existing beds, for it is incouccivable ho\y their forms could have been 

 derived from the subsequent action of any pliysical causes. These 

 masses, which could only be spread out by comparatively violent action 

 of water, or other powerful agencies, seldom contain organic remains 

 at all, and never those of a delicate structure, in perfect preser- 

 vation." 



The physical causes concerned in the production of these sediments 

 very naturally occupy the next place in our author's essay. The great 

 cause, whether due to the action of the atmosphere or of the 

 waters of the globe, is that degradation of the dry land and its shores 

 which geologists include under the term denudation. Eefore a current 

 of water can put down a quantity of matter, it must manifestly take it 

 up ; and, moreover, as water cannot while in a tranquil state either take 

 up matter or hold matter, however finely divided, in mere mechanical 

 suspension for any length of time, it follows "that the operations of 

 denudation and of deposition may be regarded as simultaneous opera- 

 tions, the rate of one being an almost exact measure of that of the 

 other. Consequently, since we have shown that the rate of deposition 

 must have been slow, we conclude that the rate of denudation must 

 have been equally so." 



The causes assigned for the denuding operations arc the waves and 

 tidal currents, the cxlialations of vapour by solar heat and its conden- 

 sation in the form of rain — causes co-existent with the moon, the ocean, 

 and the diurnal rotation of the globe ; for so long must the tides and 

 the tidal currents of the ocean and atmospheric changes have been at 

 their unceasing w^ork of abrasion and conveyance of the abraded matter 

 to lower levels. 



''It may, perhaps, be supposed that the double process of denudation 

 and deposition which we have been discussing may have been effected 

 in former periods with greater rapidity by the aid of those great con- 

 vulsive movements of the land to which many geologists are disposed 

 to refer the apparent dislocation and highly iuclined positions of so 

 many sedimentary beds. We may doubtless conceive repeated processes 

 of demolition, transport, and deposition of large quantities of matter 

 to have taken place in this manner by paroxysmal efforts ; but this 

 sudden deposition of materials in large quantities could never form 

 regularly stratified beds, or admit of the regular distribution of organic 

 remains throughout the deposited mass ; " such tumultuous transport 

 being utterly different from that which accompanied tlie deposition of 

 the ordinary sedimentary beds; and, exceedingly slow as these abrading 

 and remodelling processes now are, there is no reason for supposing them 

 to have acted with greater vigour in geological periods." 



Having thus dwelt on the origin and manipulation of tbe materials 

 forming the stratified crust of the globe, the author follows on with 

 observations on the manner in y/hich these strata were deposited, 

 namely: — That the original position of these sedimentary heds ivas 

 xerij apirroximate^y horizontal.'' He afterwards discusses the nature 



