- Notes on American Geology. 939 
This is at the same time the lightest 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, 
‘Tt 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 Enurypterus 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 
imto the mountain or carboniferous limestone ; but the Eurypterus 
is limited 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 paleontology 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 
taces 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 paleontology 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, Sone: rise to debacles, of which we 
* Agassiz, Edinburgh New Philosophical Sourael, April, 1838. 
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