CHALK MOUNTAIN. 195 



reaching the gorge a few feet below the falls, and then curves sharply to 

 the east, spanning the chasm with a picturesque bridge, and, emerging 

 through a 66-foot cut in the ridge beyond, reaches the south slopes of 

 Chalk Mountain completely above the Chalk Ridge. 



Between the valley of Chalk Creek on the west, the Arkansas Valley 

 on the south, and the head of Ten-Mile Creek on the east, is a table-topped 

 mountain of rudely triangular shape, presenting steep escarpments to the 

 south and east. This low mountain mass, which forms part of the con- 

 tinental divide, is also formed of a very light-colored eruptive rock, and has 

 from this fact received the name Chalk Mountain. Both the rock of Chalk 

 Mountain and that of the white ridge below are rhyolite, but of somewhat 

 different types. The former is coarse in texture, and, though seen to be 

 distinctly porphyritic when closely examined, it seems in some cases almost 

 granitic in structure. It is of the variety known as "Nevadite." Upon the 

 southern and northwestern edges of the plateau of Chalk Mountain the 

 surface is strewn with huge blocks of Nevadite, in which the large sanidine- 

 crystals, with their brilliant satiny luster, and the dark smoky quartz 

 crystals are especially noticeable. Over the remainder of the surface the 

 rock is somewhat finer grained, the quartz crystals being the most prominent 

 constituent. 



The rock of Chalk Mountain is different from that of all the neighbor- 

 ing bodies, not only in the character of its constituents, but in the time and 

 manner of its eruption. It has disturbed the adjacent strata to an extent 

 not noticed in connection with any other eruptive of the region, and by 

 cutting off bodies of Eagle River and other porphyries its later age is 

 proven. The masses of debris upon the steep southern slopes cover its 

 contact with the sedimentaries, but upon the east, north, and west numer- 

 ous outcrops appear, which illustrate the disturbing influence of the Nevadite- 

 mass. The map shows an arm of the Weber Grits projecting up the eastern 

 slope to the level of the plateau, and in these elevated beds is a dike of 

 older quartz-porphyry, cut off to the west by Nevadite. Fig. 1 of Plate 

 XIX shows this relation in detail. The strike of the beds is indicated by 

 the lining on the sketch, and the easterly dip measures about 70 at the 

 extremity of this arm, declining to 30 at the edge of the cliff. South of 



