WHITE.] INTRODUCTORY REMARKS. 417 



Cretaceous period and beginning of the Laramie, changing the area thus 

 inclosed to a brackish-water sea, in which the strata that we now call 

 the Laramie Group were deposited. By the continued elevation of the 

 continental area that sea became much reduced in size and entirely 

 fresh at the close of the Laramie period. * 



During the immediately succeeding Eocene Tertiary epoch at least, 

 the great fresh-water lakes that were thus formed prevailed_over a large 

 part of that area which in the Laramie period had been occupied by 

 brackish, and previously by marine, waters. Then began the series of 

 movements in the earth's crust which resulted in the elevation of the 

 plateaus and the great systems of mountains of Western North America, 

 into the structure of which these Laramie and Eocene strata enter. 

 Some portions of the western part of the continent continued to be occu- 

 pied by fresh.water lakes of the kind last referred to, during the middle 

 and latter portions of the Tertiary period ; but they were much less in 

 size than those which previously existed. They also gradually became 

 smaller, and finally disappeared by being drained of their waters ; or 

 remnants of them remained to become the salt- water lakes of to-day. 



The incompleteness of that portion of the geological record which is 

 furnished by the fossil remain's of the three categories of mollusks, which 

 form the subject of this article, has already been referred to, and the 

 causes of it are very apparent when it is remembered how small a pro- 

 portion the non-marine have always borne to the marine mollusca; and 

 also how small a proportion of fresh and brackish water deposits there 

 must always have been in comparison with marine deposits. 



The extensive fresh and brackish water deposits of Western North 

 America are remarkable exceptions to the general rule, that extensive 

 geological formations are of marine, or open sea, origin; and we have 

 therefore in that region, and for the epochs which those formations rep- 

 resent, an unusually full record of non-marine and terrestrial life; for it 

 must be remembered that those formations contain many remains of 

 terrestrial vertebrates, and an abundant flora, as well as of fresh-water 

 and land mollusca. This statement of facts naturally leads to a brief 

 consideration of the conditions which prevailed in former geological pe- 

 riods, and which conduced to the preservation of the molluscan forms 

 herein discussed, when so large a proportion of their kinds in other 

 parts of the world were destroyed. 



While the remains of aqueous mollusca were readily entombed and 

 preserved in the sedimentary deposits of the waters in which they lived 

 (which deposits afterward became rocky strata), those of land mollusca 

 must have been transported from the land into such waters, where alone 

 they could have been preserved, and where in fact they did receive the 

 same entombment with those that had lived there. This transportation 

 of the shells of land mollusca was doubtless in most cases effected by 

 the currents of rivers near the banks of which the mollusks lived, and 

 into the waters of which they were swept in time of flood. And yet an 

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