Notes on Americmi Geology. 239 



This is at. the same time the hghtest and most indestructible of 

 the cretaceous shells, and therefore the one most likely to be car- 

 ried unbroken with the detritus of the green sand. 



It is very evident that a change of the mean temperature of the 

 crust of the globe has exerted a marked agency in the destruction 

 of one group of animal life and the creation of another ; and it 

 may be owing to this cause, that the higher the organization, 

 the more limited in the geological series are the fossil remains. 

 Thus the polyparia have a higher range than the testacea, and 

 the latter than the trilobites, whilst the Eurypterus is still more 

 limited. The polyp, Cyathophyllum ceratites dates its existence 

 with the lower portion of the Trenton series, or lower transition, 

 and extends throughout all the calcareous formations above, even 

 into the mountain or carboniferous limestone ; but the Eurypterus 

 is hmited to a very insignificant portion of a single formation. 



The fall of temperature has not, as some geologists supposed, 

 taken place gradually since the creation of the globe ; but every 

 phenomenon in palaeontology goes to prove the existence of a cer- 

 tain mean temperature during a long period, and a sudden dimi- 

 nution of heat at particular epochs.* The change of groups of 

 marine animals was not produced or accompanied by any convul- 

 sion, powerful enough to cause a violent rush of the oceanic wa- 

 ters, as the fossils of one period rest upon and even intermingle 

 with those of an earlier date, as if both had lived and died on or 

 near the spots where they are now found. The theory of period- 

 ical refrigeration alone can explain the sudden extinction of whole 

 races of animals and vegetables. On the supposition that such 

 change had resulted from uplifts, which to be reconciled with the 

 facts, would necessarily have been sudden, a violent movement of 

 the waters would have torn up the surface after such uplift, which 

 has not been the case ; besides, the uplifts would have been each 

 extensive as the globe itself; an hypothesis at variance with all 

 the phenomena which palaeontology and the relative position of 

 strata present to our daily observation. Uplifts in great numbers 

 have taken place, and many of them were no doubt gradual, as 

 must necessarily have been the case where they resulted from 

 crystallization of the earth's crust : others have been sudden, pro- 

 duced by volcanic agency, giving rise to debacles, of which we 



" Agassiz, Edinburgh New Philosophical Journal, April, 1838. 



