^^.,,T GEOLOGY SOUTH PAEK. 213 



from which the hills boundiug the park on the west rise. lu the south 

 end of the park are numerous isolated buttes, the majority being of 

 volcanic origin, although some are composed in part of sedimentary for- 

 mations. They will be referred to again in subsequent portions of the 

 chapter. Almost the entire southern end of the park is volcanic, and 

 the country gradually rises from the South Platte Eiver to the divide, 

 and then slopes to the Arkansas River. The outlet of the lake existing 

 here in early Tertiary times was probably in this direction. The erup- 

 tive forces acting after the deposition of the liguitic sandstones elevated 

 this part of the country, and the water had to find an outlet in a new 

 direction. It was then that the present drainage was i)robably out- 

 lined. The lake was most likely drained through the Platte caiion at 

 the southeast end of the park. The lake itself must have extended 

 farther to the southward than the present outlines of South Park would 

 seem to indicate. The mountains on the east side of the park, as we 

 saw in the last chapter, are composed of schistose rocks which extend 

 westward some distance into the Park. The outline is rather irregular, 

 but the general course of the range is northwest and southeast, and, as 

 we have seen, the dips are to the northeast and southwest. The range 

 is therefore an anticlinal axis, which, as we go south, bends to the east- 

 ward. The elevation is over 10,000 feet above sea-level and over 1,500 

 feet above the park. The northeast boundary of the park is a continu- 

 ation of the schists, identical with those on the east except that here the 

 hills are capped very frequently with eruptive material, mostly trachyte. 

 On the more uniform level of the park, the sandstones are but little dis- 

 turbed. As we shall see further on they are of Tertiary age. The vol- 

 canic rock extends around the northern edge of the park, close to it, not 

 extending far to the northward until we get around to the northwest. It 

 seems to have been an overiiow, as I found in several places that it 

 capped hills whose bases were composed of dark micaceous gneiss. The 

 hills that rise above to the northward are also metamorphic. On one of 

 the branches of Michigan Greek, extending some distance up the valley, 

 is a tongue of sandstone, a j)rolongation from those in the park, proba- 

 bly outlining an old bay. The upper edge of this narrow belt, bounded 

 on one side by trachyte, and on the other by granite, was found by Mr. 

 Gardner to be 500 feet above the general level of the park. A period of 

 elevation in the range north of the jiark, therefore, must have taken place 

 after the deposition of this sandstone. The granites and schists form the 

 basis of the range north of the park until we get to Mount Guyot, when 

 eruptive rocks are seen at the head of Michigan Creek, near Mount 

 Guyot. This mountain has an elevation of 13,389 feet, and is on the 

 continental divide, which, on the west side of the Arkansas, has a di- 

 rection nearly north and south, and then turns to the east around the 

 head of the river, and again, beyond Mount Guyot, turns abruptly to the 

 northward toward Gray's Peak. Just east of the mountain is a low sad- 

 dle, forming the water-divide between Michigan Creek and the waters 

 of Snake Eiver, one of the tributaries of the Blue Eiver. 



On this saddle (Georgia Pass) we have the following section, shown 

 inFig. 4, Plate YI: 



(«.) 1. Erupture granite forming the peak. 



(h.) 2. Hard gray slate. 



(c.) 3. Quartzite. 



{d.) 4. Coarse gneiss, light colored. 



(e.) 5. Black micaceous gneiss. 



This latter, (No. 5,) as far as could be ascertained, extends eastward 

 for some miles. Bed No. 4 is very coarse, with white feldspar and a silvery 



