WIDE-SPEEAD UNIFOEMTTY. 



87 



harder metamorphic rocks, but it generally consists wholly of quartz grains 

 In texture it varies from an almost incoherent mass of silicious grains, 

 crumbling on exposure to the weather and easily crushed in the hand, to a 

 tolerably dense and compact sandstone, which in weathering forms cliffs. 

 Sometimes it passes almost into a quartzite, the silicious grains being- 

 bedded in a bright glassy silicious cement. The most friable varieties are 



light in color and have usually a silicious cement, 

 while the red or brown colored, in which the cement- 

 ing material is largely oxide of iron, are usually more 

 compact and durable. It is in places somewhat argil- 

 laceous, and has there a shaly character. 



Its color is also quite diverse, varying from a dark 

 reddish brown, like that of the Triassic red sandstone 

 of the Atlantic coast, to a light brown and in places a 

 pure white. The white variety is usually coarse and 

 saccharoidal ; it is found near the top of the forma- 

 tion, and is well exposed in the northern part of the 

 Hills on Bear Butte Creek and around Warren 

 Peaks. 



Though the sandstones are coarse in texture they 

 carry in many places abundant remains of fossil forms 

 in a most excellent state of preservation, and the col- 



Fig. 11.— Section of the 



Potsdam on lower Spring lections made in the Black Hills rival in richness and 



Creek, with basal por- 

 tions of the Carboniferous, in the perfection of the specimens the collections from 



better known and more thoroughly studied localities of the formation. 



Complete sections of the group from base to summit are rarely met 

 with, both because of its concealment by the broken fragments and talus 

 of the Carboniferous above, and by reason of the soft and destructible 

 character of its own beds. In the eastern canon of Spring Creek, however, 

 a most excellent section is exposed and was carefully measured by Mr. 

 Jenney. It is illustrated by Figure 11, in which the junction of the Pots- 

 dam with the Carboniferous and with the slates is also well exhibited. 



