THE NORTH PAKK. 119 



white hills, which have probably attracted the attention of all visitors to the 

 North Park, whether for pleasure or exploration, on account of their striking 

 contrast with the surrounding country. They are found to consist of loose 

 quartz sands, collected here in a somewhat remarkable manner by the pre- 

 vailing west winds. These sand-dunes rise from 100 to 200 feet in height, 

 and, although constantly changing in form, are, from the position of the sur- 

 rounding hills, kept together with but little variation in their main outlines. 



Volcanic Rocks. — Along the east wall of the Park, from Elk Camp 

 nearly to Good Pass, which goes over into Middle Park by the way of 

 Willow Creek, the lower spurs and foot-hills of the Medicine Bow Range 

 are entirely covered by immense flows of volcanic rocks, chiefly rhyolites. 

 The central point of eruption would appear to be on the slopes of Mount 

 Richthofen, near the change in strike of the Archaean rock-masses. Here 

 they lie somewhat higher than at any other point, and the flows of dark- 

 colored rhyolites contrast in a marked manner with the gray Archaean 

 body. Long volcanic flows send out low ridges into the Park, 300 or 400 

 feet in height, between which numerous streams have cut broad valleys, 

 more or less filled with coarse, detrital accumulations. These ridges pre- 

 sent a rough, broken surface, and are for the most part covered with timber. 



In their lithological aspect, they offer, so far as examined in the field, 

 but little variety in texture and composition, and over wide areas present 

 the same general features. They are, however, characteristic rhyolites, and, 

 from their position as the most easterly outburst of a rock, which plays so 

 important a part in the Tertiary volcanic phenomena of the Western Cor- 

 dillera, deserve some special mention. They are usually composed of a 

 fine-grained groundmass, in which small fissured crystals of sanidin, and 

 broken grains of black and dark gray quartz, and occasionally black horn- 

 blende are imbedded. They also carry the globular grains, or sphserolites, 

 which are often so marked a feature in rhyolites. In color, they vary from 

 reddish to purplish-gray. At the head of Sioux Creek, rhyolites of a some- 

 what different type occur, the groundmass being more compact and lighter 

 in color. The enclosed feldspars are larger, giving the rock more the appear- 

 ance of a porphyry, and include opaque orthoclase crystals, as well as sani- 



