B. Silliman—Mineralogical Notes. 31 
map. 
At the east (near the border of the map) the region abruptly 
ends along a nearly north-and-south line, the massive spurs 
falling to the zone of “Hog Backs,” or ridges of upturned 
sedimentary rocks, which lie all along the base of the range. 
The “red beds,” probably of Triassic age, form the inner- 
most ridge, lying directly on the Archean rocks of the moun- 
tains. These, in going eastward, are followed by the upturned 
edges of Jurassic shales, the Cretaceous groups, and the great 
Lignitic formation, of as yet disputed Cretaceous or Eocene age, 
which stretches eastward, and forms the beds directly under- 
lying the Great Plains. 
Boulder City is on the border bet Pp 
and is reached by railroad, Denver City being but twenty- 
five miles to the south and east. From Boulder City, wagon 
+1 Page 
. 
from fifteen to twenty miles, is Clear Creek, much like the 
occur ; the gneiss, with possibly granite, in the greater propor- 
tion. While large areas of structureless granite abound, ap- 
parently of so-called plutonic or eruptive origin, search seldom 
fails in finding spots or areas more or less large of gneissic or 
even distinct schistose structure. The fact that these usually 
merge imperceptibly into the surrounding granite, as well as 
conform in their strikes and dips to the general system of folds, 
as more plainly indicated perhaps in adjacent schistose regions, 
show that such granites have been metamorphosed in site and 
indigenous rocks. At the same time sharp lines of 
