CAMBRIDGE ESSSAYS : HOPKINS* "GEOLOGY.'* 
431 
dominence of this character. There are some beds, however, which 
coasist principally of rouaded pebbles, or other masses of various sizes, 
which must have had their present forms whea first arranged in the 
existing beds, for it is inconceivable how their forms could have been 
derived from the subsequent action of any physical 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. Before 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 mattei-, 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 wo 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 are the waves and 
tidal currents, the exhalations 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 work 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 inclined 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 difi'erent from that which accompanied the 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 the materials 
forming the stratified crust of the globe, the author follows on with 
observations on the manner in which these strata were deposited, 
namely : — " That the orir/inal position of these sedimentary beds was 
very apirroximateiy horizontal." He afterwards discusses the nuturo 
