THE NORTH PARK. 125 



A specimen of tliis trachyte was subjected to chemical analysis by Mr. 

 R. W. Woodward, with the following- result: 



Sihca 61.95 61.95 



Alumina 16.75 15.80 



Ferrous oxide , 5.53 5.76 



Lime 4.24 4.24 



Magnesia---- - - 2.54 2 63 



Lithia l trace trace 



Soda 441 4.50 



Potassa--- --- 3.48 3.51 



Water-- 1.22 1.34 



100.12 99.73 

 Specific gravity, 2.7, 2.6. 



In this trachyte are enclosed fragments of unaltered, fine-grained gran- 

 ite and dark hornblende-rock. 



Other trachytes from this same region present much the same physical 

 habit, with the same mineral constituents, varying chiefly in the compact- 

 ness of the rock and in color. 



The basaltic formation, which enters into the structure of the divide 

 between the two Parks, lies almost entirely westward of the trachyte region. 

 It rarely attains the elevation of the older outbursts, but presents a more 

 broken, irregular surface, with several outlying buttes and hills. These 

 hills form a striking feature in the scenery, rising from 200 to 700 feet above 

 the level of the Park basin, with a crater-like shape, that recalls the small 

 cones which are frequently seen smTOunding the base of large volcanoes. 

 That they are connected a short distance below the surface vvdth the main 

 mass of basalt would seem highly probable, but are now separated by hori- 

 zontal Tertiary strata, which abuts up against then' steep slopes. 



Buffalo Peak, just east of the west fork of the Platte, is the most prom- 

 inent of these basaltic liills, rising nearly 700 feet above the stream-bed 

 below, while its summit, a level sm'face, measures only some 300 feet across. 

 The Hthological character of all these basaltic hills is quite similar, being 

 normal fine-grained basalts, varying from black to dark gray, a7:id, with the 



