. GENERAL OBSERVATIONS. 87 



By comparing the invertebrate fauna of those fresh water Tertiary 

 deposits with the faunae of existing fresh waters we observe the entire 

 absence from our collections of all articulates, except a species of Cypris 

 and the insects discovered by Mr. Cleburn. The list of Molluscan genera 

 found in those ancient lakes compares closely with those of the great existing 

 American lakes, notwithstanding the fact that the floral remains of the 

 former indicate a uniform and much milder climate than now exists in either 

 of the two regions. 



Thus far no well defined fluvatile deposits have been discovered, but it 

 is probable that those fresh water species found among brackish water forms 

 constituted portions of the molluscan faunae of rivers that flowed into the 

 ancient lakes or estuaries, because they are specifically different from, and 

 more various than, those of the purely fresh water deposits. This sugges- 

 tion is supported by the well-known fact that the molluscan types of lakes are 

 fewer than those of rivers, while the differentiation is yet greater in brackish 

 and greater still in marine waters. 



In closing these remarks upon the Ternary period a question arises 

 similar to the one briefly considered at the close of the remarks upon the 

 Cretaceous period, namely : Why has the dividing line between the strata 

 of the Tertiary and Cretaceous periods been drawn where it is rather than 

 at some horizon either above or below it ? 



The answer is nearly the same as in the former case There is no 

 physical break in the Cretaceous strata, from the base of the series to the 

 top of the upper, or Point of Rocks Group, at which horizon there is at all 

 observed points, extending over a large region, considerable unconforrna- 

 bility by erosion of the lower strata of the Bitter Creek Group, upon the 

 upper strata of the Point of Rocks Group. 



The separation of the two periods, as represented by the strata of the 

 .Plateau Province, is a physical rather than a paleontological one. Upon 

 purely paleontological ground it is difficult to indicate precisely where the 

 line should be drawn, but it should evidently be somewhere near the one 

 indicated in Professor Powell's section. This being the case, and it being 

 necessary to draw such a line, it is more rational to draw it upon a line of 

 a physical break than through conformable strata either above or below it. 



