INTERBEDDED QFARTZITES. 



91 



feet — resting unconformably on the Archaean, and underlying Silurian and 

 Carboniferous limestones ; and this is considered by King, Hayden, and 



Bradley to be Potsdam,* though no fossils are found 

 in it. The Rocky Mountains of Colorado also con- 

 tain in many places similar quartzites with the same 

 relation to the older rocks. Peale has studied them 

 in the Park range, and besides the basal quartzites 

 here alluded to finds other beds intercalated with the 

 sandstones he regards as Potsdam. In a continua- 

 tion of the Organ Mountains, near El Paso, Tex., 

 Jenneyf found a quartzite 25 feet in thickness, rest- 

 ing upon the granite and overlaid bj beds of Potsdam 

 well characterized by fossils. 



These citations suffice to show the wide-spread 

 development in the Rocky Mountains of quartzites 

 at the base of the Paleozoic system, and the una- 

 nimity with which they have been referred by the 

 best authorities to the Potsdam sufficiently warrants 

 the same reference of the basal quartzites in the 

 southern part of the Hills. 



The quartzites found in the body of the forma- 

 fig. 13— Section of the Pots- tion differ somewhat in their character from those just 



dam on Box Elder Creek. , 



described. Ihey were best observed and studied on 

 Box Elder Creek in the section illustrated by Fig- 

 ure 13. 



On the same creek, some eight miles farther west, 

 a cliff of red sandstone 30 feet in height at the 



3 :T5s= 



1. Slates inclined at a high 



angle. 



2. Coarse sandstone and 



gravel conglomerate. 



3. Broad talus, no exposure. 



4. Cliff of coarse red sand- 



stone with interstrati- 

 fledlayers of dense,hard, 

 deep red or purplish 

 quartzite. 



5. Talus with fragments of top of the formation contains six or eight strata of 



the shaly impure lime- 

 stone of the Carbonifer- hard, purplish, glassy quartzite from 6 inches to 1 foot 



6. Carboniferous limestone. jj^ tllickllCSS. 



Intercalated quartzites are also seen on French Creek where the cold- 

 is yellowish brown, and the rock seems to be little more than a fine quartz 



* Mining Industry, 40th Parallel Survey. King. Chap. vi. Geological Survey of the Territories, 

 1871. Hayden; p. 15; Ibid., 1872. Bradley, p. 194. 



t American Journal of Science. 3d series, vi, 1874, p. 25. 



