454 DESOEIPTIVE GEOLOGY. 



to the south are formed of the same quartzites, standing almost perpendic- 

 ular. From the limestones of these eastern hills were obtained 



CJionetes granulifera. 

 ^~~Productus Nehrascensis. 

 Productus longispinus. 

 Martinia lineata. 

 Athyris subtilita. 



In the steeply-dipping limestones at the northeastern point is an interest- 

 ing cave, over 200 feet in length and in places as much as 25 feet high, whose 

 walls and roof are covered with a sort of tufaceous conglomerate, made up of 

 pebbles of dark-blue limestone, cemented by a white calcareous tufa, while a 

 similar tufa covers the floor. These are the remnants of one of the lower 

 lake-terraces. In them were found a few recent shells of the genus Amnicola. 



Along the steep front of the point of the mountains, which has an angle 

 of slope of nearly 37°, the old lake-terraces can be traced with great distinct- 

 ness, the upper one being marked by a line of calcareous tufa clinging to rock 

 slopes. A careful measurement was made of the height of this terrace-line 

 above the level of the present waters of the lake by a series of synchronous 

 barometrical observations. The result gave 940 feet. The little islet about a 

 mile from the shore, which gives the name to this point, is composed of 

 limestone strata, lying nearly horizontal, whose weathered surfaces have 

 become quite black. 



TooELLE AND RusH Valleys.— The broad plains to the west of the 

 Oquirrh Mountains are divided by a low ridge, called the Stockton Hills, into 

 two distinct valleys. Tooelle Valley, to the north, is a continuation of the 

 present basin of Salt Lake, and on the borders of the lake consists largely 

 of marsh-land and swamps, while along the stream-beds of the higher por- 

 tions of the valley are considerable stretches of arable land occupied by the 

 farms of Mormon settlers. On the flanks of the mountains, especially in 

 the southern portion of the valley, the old lake-terraces are very distinctly 

 marked, and at either end of the Stockton Hills form a broad, flat ridge, 

 separating the two valleys. 



Rush Valley, as the northern portion which is cut off by these lake- 



