664 



A FEW REMARKS 



that those shells have undergone enormous pressure beneath 

 an ocean, when they were surrounded with mud.* But pre- 

 vious to such pressure, the shells must have grown naturally 

 somewhere : — certainly not at the bottom of an ocean ; because 

 they are shells of a comparatively delicate structure which are 



* On this subject, the pressure of an ocean, Mr. Lyell remarks, 

 (Elements of Geology, 1 838, pp. 7, 8, 9.) " When sand and mud sink to 

 the bottom of a deep sea the particles are not pressed down by the 

 enormous weight of the incumbent ocean ; for the water, which becomes 

 mingled with the sand and mud, resists pressure with a force equal to 

 that of the column of fluid above." " Nevertheless if the materials of 

 a stratum remain in a yielding state, and do not set or solidify, they will 

 be gradually squeezed down by the weight of other materials successively 

 heaped upon them, just as soft clay or loose sand on which a house is 

 built may give way. By such downward pressure particles of clay, sand, 

 and marl may become packed into a smaller space, and be made to cohere 

 together permanently." 



" But the action of heat at various depths is probably the most power- 

 ful of all causes in hardening sedimentary strata." 



In refliecting upon these passages it appears to me that Mr. Lyell has 

 supposed what may not always take place in a deep sea, namely — that 

 sand and mud sink to the bottom. 



Whenever particles of sand and mud are at the bottom, they must be 

 lower than contiguous particles of water, or they could not be at the 

 bottom ; therefore those particles of sand and mud have water above, while 

 resting upon some other substance below. Pressure there can be none, 

 excepting of some earthy particles upon others, while the specific gravity 

 of the sand and mud exceeds that of the displaced fluid. But, if the 

 depth of water be increased, and its specific gravity at the bottom 

 augmented, the sand and mud at the bottom must rise, if they do not 

 cohere together, and to the surface on which they lie ; in which case 

 the increasing weight and density of water would tend to compress and 

 make them cohere still more. 



The smaller kinds of sea shells are very little heavier than sea water. 

 This would prevent their being carried by the action of the sea to great 

 depths, even if it were possible for them to be so rolled over rocks, sand, 

 or mud, in which they would stick, or be buried, before they had been 

 moved many miles from the place where they grew. These two con- 

 siderations may help to account for the fact that seamen do not find im- 

 pressions of shells, on the * arming' of the lead, when sounding in very 

 deep water, at a considerable distance from any shore where they grow. 

 Sea-shells, I need hardly remark, grow only in comparatively shallow 



water. 



