NORTH PARK, COLORADO. • 199 



ed by lofty mountains, reaching up 3,000 feet above the inclosed valley. It is 

 covered chiefly with a dense growth of wild sage [Artemisia). 



On our route from any point we can see snow-clad mountains on nearly 

 every side. These snowy peaks impart a pleasant coolness to the various waters 

 flowing through the valley. The water power of the streams flowing out from the 

 mountains is sometimes great, that of Jack's Creek especially so. Its width fifteen 

 to twerity-five and thirty feet, flowing very rapidly, falling in many places five to 

 ten feet per 100 feet or 200 to 300 feet per mile on its mountain course. Strange 

 as it may appear these streams contain but few fish, but just across the "Conti- 

 nental Divide " fish are abundant. 



GENERAL GEOLOGY. 



The Colorado Range and the Black Hills are archean. The Medicine Bow 

 Mountains and the main Continental Divide are also archean. Through these 

 archean rocks, igneous rocks of volcanic origin have pushed themselves up. 

 Clarence King informs us (in Geol. Survey of 40th Parallel) that those on the 

 south rim of the Park are trachytic, those flanking the western slope of the arch- 

 ean on the heads of Michigan River and Jack's Creek, he terms " Rhyolite," and 

 further says: "From the meridian of 114 W. to California the Rhyolitic rocks 

 ' cover a greater area than any other of the volcanic family, and in age are Post- 

 'miocene, and were characteristic of the opening of the Pliocene." Mr. King 

 defines Rhyolite as " a ground mass of fine-grained mingling of fragmentary crys- 

 ' tals of Sanidin (glassy feldspar) and crystalline grains of dark quartz, the color 

 ' generally dark. At the head of the Illinois Creek the ground mass is lighter, 

 ' and includes larger crystals of feldspar and fragments of quartz. Hornblende 

 * also occurs in small crystals. The middle of the ridge south of North Park is 

 ' of Trachyte, which Zirkel calls a granite-phorphyry. The east wall of the Park 

 ' is lined with Rhyolite. Basalts occur west of the Trachyte. Rabbit's Ear Moun- 

 'tain and Buffalo Peak are of Basalt." — King, 40th Parallel Survey. 



King refers the chief formations of the North Park basin to the Tertiary, 

 leaving exposed at several places near the outer rim the underlying cretaceous. 



The above, from King's Report, includes about all that has been written con- 

 cerning the geology of this interesting region. Other surveys seem to have pass- 

 ed it by, but we infer that there were great disturbances and eruptions during 

 the development of these trachytes. 



Mt. Richthofen (probably the same as Lead Mt.) stands at the point of 

 meeting of two distinct trends of the Rocky Mountain archean rocks. Within 

 this angle occurs an extensive outpouring of rhyolite rocks ; they flank the base 

 of the archean for twenty-five miles, rising highest against Mt. Richthofen, when 

 the volume of eruption was the greatest. In this vicinity the granitoid rocks are 

 deluged by dark colored rhyolites. 



Virginia City, Nevada, and the adjacent mines are on or near the extreme 

 western extension of the great Rhyolite overflow; the southern and southeast 



