10 



complish the great excavations that now show the former bottoms of the basins. 

 These and like points future investigation will undoubtedly solve. 



It is certain that the level of the lakes varied at different times, and also 

 that great stretches of marshy land surrounded their borders. 



The first fact is proved by the characters oft he different layers of strata ; the 

 second by the fossil remains entombed. A careful study of the formation of 

 the beds of Cottonwood Creek, at a point about fourteen miles south of Fort 

 Bridger, yielded the following result, which will serve as an example illustra- 

 tive of the regular formation in this section. 



Three distinct lines of bluffs are to be noticed, the first rising to a height of 

 one hundred and fifty feet ; from these extends a plain, gently sloping south-west 

 to the foot of the second line of cliffs; these seem high, owing to the downward 

 slope of the plain, but they really rise only fifty feet higher than the first. 



On the top of the second, but less broad than the first, extends a level plain, 

 with a slight dip to the south-east ; at the end of this, the third line rises two 

 hundred feet above the top of the second line of bluffs, making a total height, 

 in the series, of four hundred feet above the level of Smith's Fork. 



The strata throughout are nearly horizontal, and are of different color and 

 composition. 



Specimens of the rocks and clays from this section were gathered, and sub- 

 mitted for analysis to Professor Cornwall, of the Scientific School at Princeton, 

 who has kindly furnished us with the following notes : 



No. i, very friable, light greenish-gray sedimentary rocks, consisting chiefly 

 of crystalline grains of quartz, orthoclase, and hornblende (this often in slender 

 crystals), with a little dark mica, and irregular fragments of a light-greenish, 

 transparent, not dichroitic mineral. The above are not perceptibly affected by 

 hot hydrochloric acid. The whole is loosely cemented with a calcareous clay, 

 containing considerable phosphoric acid. These rocks might result from the 

 disintegration of a neighboring hornblendic granite. 



The greatest mass of the strata is made up of this kind of rock, and it is 

 this which gives the peculiar color to the " Mauvaises Terres." 



Above this in places is found a second kind, which is a light gray indurated 

 clay, with a slight greenish tint. It contains much fine crystalline quartz, with 

 considerable carbonate of lime, and a little phosphoric acid. 



It appears to be of similar origin with the first, but was deposited in quieter 

 waters. This mineralogical evidence is strengthened by the fact that no re- 

 mains of mammals were found in strata of this kind, but only shells regularly 

 deposited in layers one above another. 



The third kind is found in thin layers, overtopping the highest line of buttes ; 

 it consists of very fine-grained dark-brown sandstones, containing a consid- 

 erable proportion of carbonate, and phosphate of lime. They are hard and 

 tough, and are mechanically deposited, and no fossils are found in them. 



In No. 2 the indurated clay is often found above a coarser sandstone than 

 No. i, but of the same general appearance ; with the exception that it contains 

 smooth, rounded pebbles, which were deposited either on a beach or in running 

 water. In this stratum the fossils found are separate bones, often showing 

 marks of having been broken before they were silicified. This would prove 

 that the lake level was changing continually. 



