PEOSPECT MOUNTAIN. 185 



there are many prospect holes, showing the character of the I'ock beneath 

 the covering- of debris, by means of which the general outlines of its struct- 

 ure can be determined. These are shown in Section C, Atlas Sheet VITI, 

 and in Section K, Atlas Sheet X, the former of which follows the crest of 

 the ridge in an east and west direction, while the latter crosses it in a north- 

 west direction. P^-om these it is seen that from the summit of Prospect 

 Mountain eastward to the Mosquito fault the surface is occupied by beds 

 of the Weber formation, with a general easterly dip. They are crossed by 

 a few dike-like masses of eruptive rocks, the most jn-ominent of which are 

 two dikes on the crest of the ridge : the one a coarse-grained quartz-por- 

 phyry, with large crystals of orthoclase, which resembles a Gray Porphyry; 

 the other a fine-grained micaceous rock resembling a diorite, but yet con- 

 taining a large proportion of orthoclastic feldspar. The latter rock also 

 occurs on the north slope of the massive near the head of Indiana gulch, 

 and in an important body at the mouth of Bird's Eye gulch where it de- 

 bouches into the East Arkansas Valley. West of the summit of Prospect 

 Mountain, however, the slopes toward the adjoining valleys are very steep 

 and cut through the Weber beds, disclosing a somewhat complicated anti- 

 cline, or rather the intersection of two systems of anticlines, and the devel- 

 opment of a large body of porphyry found only in this mountain and on 

 Mount Zion, which is separated from it by the deep cut of the East Ar- 

 kansas Valley. This porphyry, which has already been described as a more 

 crystalline variety of the White Piirpliyr}^ and which is designated by tlie 

 color of that rock on the map, is called, from the locality of its principal 

 development. Mount Zion Porphyry. 



Mount Zion Porphyry. — Tliis porphyry is exposed in great thickness under 

 the Weber Grits on Mount Zion and on the northeast slope of Prospect 

 Mountain; it is also denuded at the head of the north fork of Little Evans 

 by the deep erosion of the gulch. It apparently replaces in part the main 

 sheet of Gray Porphj-ry, which directly underlies the Weber Grits to the 

 north and south of it and thins out as the former grows thicker. For 

 this reason it has been indicated on the sections as a rapidly thickening- 

 sheet, though it is not at all improbable that it may be a laccolitic body, 

 like those of White Ridge and Gemini Peaks, and have its vent, or channel 



