THE POTSDAM EN DETAIL. 101 



with fragments of quartzite to within 150 feet of the capping Carboniferous 

 limestone. 



On Bumtwood Creek the coarse sandstones of the Potsdam were not 

 discovered, but the low hills near the creek are capped by a hard grayish 

 quartzite lying unconformable upon the schists. 



In the canon of French Creek a most excellent exposure of the Pots- 

 dam is seen and the section there made has already been given. The basal 

 quartzite is less prominent, but the formation contains an additional bed of 

 quartzite separated from the basal by a soft coarse sandstone. The granite 

 approaches the sedimentary rocks on Amphibious, Burntwood, French, 

 and Battle Creeks more nearly than elsewhere in the Hills, and its prox- 

 imity may bear some relation to the greater development of the basal 

 quartzite of the Potsdam in those localities. On French Creek a granite 

 mass abuts against the lower edge of the Potsdam quartzite for a con- 

 siderable distance, and its upper two or three feet are decomposed into a soft, 

 clayey material. With the rapid eastward dip at this place the Potsdam, 

 from an elevation of 200 feet above the creek, sinks in three miles below its 

 bed. 



Between Battle and Whisky Creeks a peninsula of the Potsdam extends 

 westward toward Harney Peak, consisting, so far as examined, of a hard 

 quartzite estimated to be over ,00 feet thick. With this peninsula ends the 

 basal quartzite of the southeastern part of the Hills and on Spring Creek 

 the Potsdam corresponds more nearly to its western development both in 

 character and thickness. It may be well to note in this place that the best 

 sections of the older sedimentary rocks are always exposed in the creek 

 cafions on the east side of the Hills. The vertical cliffs of those canons, 

 400 feet or more in height, are often clean walls of the composing for- 

 mations. 



The most interesting feature of the examination on Spring Creek was 

 the discovery of strata covered with casts of a gigantic spreading fucoid, 

 Palceopltycus occidentalis. The rock is a shaly, argillaceous sandstone near 

 the middle of the formation, and some of the specimens on its surface are 

 five or six feet in length, the main stem being nearly an inch in diameter 

 and dividing into numerous branches three-eighths of an inch in diameter. 



