600 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



strata yet discovered, limestone is exactly that wbich, by the 

 regularity and continuity of its beds, l)y the extreme perfection 

 of its organic contents and by the absence of proofs of mechanical 

 action, gives most completely the notion of a chemical precipitate. 

 It appears sufficiently probable, in several instances, that the quan- 

 tity of limestone deposited in a given geological j^eriod was least 

 toicard tJie shores, and greatest toivard the deep sea; exactly the 

 reverse of what happens with the mechanical deposits of sandstone 

 and shale; it may therefore be viewed as an oceanic deposit result- 

 ing from a decomposition of sea water, aided in many instances to 

 a wonderful extent, by the vital products of zoophitic and molus- 

 cous animals." 



Page 65 he says: "The deposition of limestone by chemico- 

 vital precipitation would probably happen over a larger portion 

 of the bed of the sea, and be abundant in proportion to the depth, 

 clearness and tranquilitg of the lualer; hence strata of limestone 

 rcould thicken toiuard the center of the oceanic basin. They would 

 be of more uniform texture, and perhaps of purer composition, in 

 that direction." 



Pajre 50 : "The attentive observer soon learns to consider the 

 operations by which sandstones and clays were accumulated, as of 

 short duration, and intermitting action; while the production of 

 limestone is reijarded as the result of one continuous and almost 

 imintcrrupted series of chemical changes. 



" The carboniferous system in South Wales, which is principally 

 limestone, is more than two and a half mile in thickness." 



If any critic still insists that the weight of the sediment is insuf- 

 ficient to account for depressions of the earth's crust, and prefers 

 the theory of Leibnitz, that the radiation of heat caused the inter- 

 nal lava to contract and the external crust to fall by its own 

 weight; I reply, that both theories may be true, since one of them 

 does not necessarily exclude the other. We may admit that the 

 internal molten lava did cool and shrink so that the crust fell 

 down upon it, and then ask — would it not be certain to fall, in 

 preference, in those places where the oceanic sediment added most 

 to its weight ? 



When to this consideration we add the fiict that the elevations 

 and depressions coincide with the ocean currents, is not the proof 

 conclusive ? 



Professor R. P. Stevens, followed' Mr. Grimes, in opposition to 

 the novel theory which had been elaborately and ingeniously put 



