92 DESCEIPTIVE GEOLOGY. 



About two miles east of Como Station, in the cream colored marls, 

 were found a few small Lamellibrancliiate shells, but not sufficiently well 

 preserved to allow of specific determination. 



It would seem somewhat remarkable, that while organic remains should 

 be so difficult to find over wide areas of exposed Jurassic strata, here 

 certain species, as Pentacrinus asteriscus and Belemnites densus, should prove 

 to be so abundant. The latter type, pencil-shaped and of a dark slate 

 color, occurs from 2 to 6 inches in length, and as they withstand atmospheric 

 agencies remarkably well, and the marls in which they are embedded erode 

 so readily, large numbers of them may be picked up along the foot of the 

 bluffs, from which they have been washed out. As shown in the section 

 given above, the well-defined Jurassic rocks have a thickness of nearly, if 

 not quite, 200 feet. Only a limited development of red sandstones is 

 exposed, consisting of yellowish-red beds, and compact mud-rocks, with 

 but little local interest. East of the lake, on a gentle slope, they exhibit 

 in a most marked manner the peculiar cracks and ripple-marks so charac- 

 teristic of Triassic sandstones. Here we have deep symmetrical grooves 

 occurring at intervals of 2 or 3 inches, for nearly 1 00 feet, covering the 

 entire exposed rock-surface with the regularity of human workmanship. 



On the north side of the lake, the Dakota Cretaceous forms a low but 

 abrupt wall along the shore, dipping to the northeast at an angle of 36° to 

 40°. At the extreme northeast corner beds of fine marl and coarse friable 

 sandstone would indicate that the Jurassic beds occurred directly under the 

 level of the lake. Along the anticlinal valley, on the east side of the lake, 

 no outcrops are visible, the surface being covered with low, irregular sand- 

 dunes; but on the opposite shore it is hemmed in by a low bank of Dakota 

 sandstone. 



Como Lake, which is probably quite shallow, occupies the anticlinal 

 valley, and measures aboiit one mile in length by half a mile in width. It 

 scarcely differs from the many other small lakes, which occur scattered over 

 the Cretaceous and Tertiary plains of Wyoming, unless it be that the sur- 

 roundings are somewhat more than ordinarily dreary and desolate. Irony 

 certainly prompted the name for this sheet of water. The water possesses a 

 mild brackish, alkaline taste, but deposits onl}^ a slightly saline incrustation 



