92 GEOLOGY OF THE BLACK HILLS. 



sandstone in which the grains are bound together by a clear, glassy, 

 quartzose cement. They occur also at other localities, but they were best 

 studied on Box Elder Creek. 



Their variations in color are the same as those of the normal sand- 

 stones of the formation. Their texture at a first glance seems very like the 

 quartzites of the metamorphic rocks, but a closer examination reveals the 

 same differences which distinguish the basal quartzites of the formation 

 from the Archaean. The quartzites of the slates and schists have a very 

 glassy and uniform fracture, and their texture is homogeneous, resembling 

 that of quartz. The Potsdam quartzites on the other hand are found by 

 close examination to consist of an aggregation of small quartz grains, com- 

 pacted together by a glassy quartzose cement, which at the same time masks 

 their minute structure. The grains of quartz are generally clear and trans- 

 parent even in the darker colored varieties, rounded like water-worn sand, 

 and often brilliantly lustrous like hyalite. In some instances the quartzite 

 weathers into a coarse sandstone, readily crumbling, from which the sili- 

 cious cement seems to have been removed by solution. The basal quartz- 

 ites are usually more compact and durable, but even the quartzite formed 

 at Crow Peak by the direct metamorphic influence of the heated volcanic 

 ejection is found to decompose in a similar manner into a coarse sandstone. 



An account of the minute structure of the quartzites will be found in 

 Mr. Caswell's report. Specimens obtained from the Potsdam are indicated 

 by the following numbers: 38, 169, 200, 201, 204, 205, 206, 207, 208, 209, 

 210, and 211. 



Quartzites similarly interstratified in the Potsdam have been noticed 

 in many places in the Rocky Mountains, but no theory has, I think, as yet 

 been propounded for their formation. If only the basal member of the 

 series assumed this character it would be easy to suppose that there was 

 some metamorphic action proceeding from below — either an igneous action 

 connected with the orographic movements by which usually the Potsdam 

 is brought to light, or else a chemical interchange between the schists and 

 the sandstones, whereby a silicious cement was supplied to the latter. But 

 no such explanation will apply to the interstratified quartzites. If we sup- 

 pose them to have been formed by igneous induration, even accompanied 



