NORTH SIDE OF HORSESHOE GULCH. 157 



siderable outcrop of White Porphyry, whose thickness may be estimated 

 "at 200 feet. Within the White Porphyry is a dark porphyry, very much 

 altered, but similar in appearance to the Sacramento Porphyry, and which 

 may once have been connected -with the body of this rock already described 

 above the Sacramento mine. These are succeeded by the Blue Limestone, 

 whose beds, as shown in the section and sketch, curve up and cover, some- 

 what irregularly, the double-pointed ridge over the arch of the fold. From 

 this limestone well-preserved specimens of Spirifera Rockymontana were 

 obtained. In the Blue Limestone on the crest of the arch are, according to 

 Professor Lakes, numerous vertical cracks, which may be cross fractures 

 resulting from folding. The lithological character of the Blue Limestone 

 varies greatly in different portions. Black chert concretions, which are as 

 elsewhere most frequent at its summit, are also found well down in the for- 

 mation. Many of the beds, especially near the base, are comparatively 

 light-colored. No satisfactory continuous section was obtained of the lower 

 Paleozoic beds, though the estimate of their aggregate thickness does not 

 vary from that obtained elsewhere. At various points an included bed oi 

 White Porphyry, near the top of the Lower Quartzite, and averaging about 

 thirty feet in thickness, was observed. The Archean is composed of gneiss, 

 and of red porphyritic granite with large orthoclase crystals. 



On the eastern slope of the anticline, outcrops of beds above the Bliu 

 Limestone are exposed in the forest-covered region near the road leading 

 from East Leadville to Spring Valley, where they are much obscured by 

 surface accumulations, and, on the steeper slopes, by the relics of a lateral 

 moraine. Above the Blue Limestone the White Porphyry can first be distin- 

 guished; next is an interval of coarse sandstone; then a body of Sacra- 

 mento Porphyry, which apparently thins out rapidly to the southward. 

 The White Porphyry, on the other hand, rapidly thickens in that direction, 

 as shown by its section on the eastern slope of Sheep Mountain. 



An attempt was made by Professor Lakes to obtain a continuous section 

 from here eastward, through Fairplay, across the upper members of the 

 Carboniferous and the overlying Triassic, Jurassic, and Cretaceous beds. 

 The result was not very satisfactory, inasmuch as a great portion of the 



