84 GEOLOGY OF THE BLACK HILLS. 



Peak it forms the surface of a considerable territory, but in other portions 

 of the Hills it is generally but an outcroping stratum in the face of a cliff 

 or slope. 



Though essentially a sandstone formation it has some variety of com- 

 position. Usually it carries a conglomerate at the base, but this is some- 

 times exchanged for a dense quartzite, and in many places there are inter- 

 stratified beds of quartzite. In a few localities the formation is quite cal- 

 careous, and in a greater number it contains peculiar greensand deposits. 

 Its thickness is quite uniform, ranging generally from 200 to 250 feet, but 

 attaining to 300 feet on the north branch of Redwater Creek. 



The basal conglomerate varies in character and thickness in different 

 parts of the Hills and even within short distances, but is everywhere a well- 

 rounded shore deposit. Even where it is replaced by quartzite the latter 

 was determined to contain quartz pebbles. The pebbles and bowlders 

 forming the conglomerate were examined with minute care, and were found 

 in all cases to consist of the harder varieties of the rocks composing the 

 metamorphic series — quartz, the hard blue and gray quartzites, and some 

 of the harder slates and schists. The latter usually occur as flattened peb- 

 bles similar to those found in the present streams. The banded or jaspery 

 quartz, ribbon quartz, ferruginous quartz, etc., of the characters mentioned 

 in the previous sections, were also found, together with a few feldspathic 

 pebbles from the granite. In the upper portions of the main conglomerate 

 and in the occasional fine conglomerates occurring in the body of the forma- 

 tion the constituent pebbles are almost entirely of quartz. 



The discovery of gold in the Potsdam was not made until after the 

 completion of our field work, nor indeed until after the major part of this 

 chapter was penned, and I am still unable to give the particulars of its occur- 

 rence. There is no need, however, to substantiate the announcement of that 

 which is inherently so probable. The currents that spread the Potsdam sedi- 

 ments worked over the debris of the veins of the Archaean, sorting coarse from 

 fine and heavy from light, and it might have been anticipated from the 

 outset (if only the. idea had suggested itself) that they would prove to have 

 concentrated the gold which then as now was scattered through the ancient 

 quartzes. 



