151] FLORA OF BOULDER, COLORADO 3 



phyry dike, and thus weathering more slowly than the 

 granitic peaks. This whole elevated plateau, cut by 

 streams into what now appear as definite mountain 

 ridges, we shall call the foot-hills, although the foot-hills 

 proper are the ridges of sandstone at the edge of this granite 

 plateau. The flora, however, is the same, save for 

 a few ferns and other rock-plants which are confined to cer- 

 tain kinds of rocks, some to the limestones, others to the sand- 

 stones, still others to the granite. 



The main range of mountains as well as the high plateau 

 at its base is composed of granite, granite-porphyry, and 

 granite-gneiss, gray or reddish in color. Dikes are frequent, 

 either of pegmatite or of felsitic porphyry. When the uplift 

 or uplifts occurred, which made the Rocky Mountains, the 

 sedimentary rocks resting upon the basement of granite, were 

 tilted until they stood nearly on end. The jagged crags of the 

 foot-hills proper are, then, the ends of these sedimentary layers. 

 Thus it happens, too, that the oldest beds lie next the granite, 

 while the younger underlie the plains. 



The oldest and lowest, that is, the one lying directly upon, 

 or rather against the granite, is a layer of quartzite 550 feet 

 thick, and of Algonkin age. This, however, is absent in front 

 of Boulder and occurs in but two places in the county. 



The next, and of Pennsylvanian (Carboniferous) age, 

 is the red Fountain sandstone, 500 to 1,500 feet thick. 

 In the immediate vicinity of Boulder it lies directly 

 upon the granite. On the east slope of Green Moun- 

 tain it hangs in five triangular blocks of about 500 feet 

 in thickness at an angle of about 52 °. These, called the 

 Flat-irons, are each about 1,000 feet high and about 1,500 

 feet wide; the third Flat-iron, however, rises to an altitude 

 of nearly 8,000 feet, or about 2,000 feet above the mesa. At 



