ASHLEY CEEEK BASIN. 293 



leys, representing the variegated clays of the lov/er portion of the Jurassic 

 formation, which are more distinctly exposed in other sections. Forming a 

 low ridge in the midst of this clayey interval, was found another deposit of 

 gypsum about 25 feet in thickness, like the other, massive and compact, 

 though of a more grayish color, and giving on analysis 76.736 per cent, of 

 sulphate of lime and 21,512 per cent, of water. Neither of these gypsum 

 deposits was found exposed for any great distance in the strike of the beds, 

 and they probably are lenticular bodies enclosed in the clayey strata. The 

 only other point at which they were found was on Sheep Creek, as already 

 mentioned; but it is not at all unlikely that they exist in many other parts of 

 the Triassic formation, since they would be easily concealed from observa- 

 tion by the disintegration of the clay beds which enclose them. A little to 

 the south of this second gypsum deposit, a low ridge of coarse glistening 

 sandstone is capped by about 50 feet of blue and drab limy slates and 

 sandstone-shales, full of highly-characterized Jurassic fossils, among which 

 were recognized — 



Gryphcea calceola var. Nebrascensis, 



Eumicrotis curta, 



Belemnites densus, 



together with two undetermined lamellibranchiate shells like Astarte. 



To the south of this ridge another intervale occurs, which the character 

 of the soil shows to be made up largely of red and purple clays, the latter 

 being seen at the foot of the face of the next ridge under a white sandstone, 

 whose base is a conglomerate, which is supposed to be that which forms 

 the bottom of the Dakota group. The thickness of the Jurassic beds is 

 difficult to estimate at this point, but seems to be greater than that observed 

 on the northern slopes. Above the white sandstone are about 50 to 100 feet 

 of blue clay-slates, the summit of the ridge being formed of compact brown 

 sandstone, in which were found the following fossils of the Colorado group: 



" Inoceramus Ellioti (?). 



Cardium, n. sp. 



Lucina or Astarte (?). 



Small mytiloid shell (?). 



Several undetermined bivalves. 



