92 GEOLOUY AXD MINING INDUSTRY OF LEADYILLE. 



only there be a sufficient thickness of loose detrital material, whether g-ravel 

 or alluvial soil, accumulated over the hard rock surfoce. Along the alluvial 

 bottoms of the streams, it is true, there is often a fringe of willow, alder, or 

 Cottonwood ; but the sturdy pine, although delighting to face the mountain 

 blasts on bare inaccessible precipices, seems afraid 1 1 trust himself where 

 he cannot thrust his roots down to a base of firm rock, or around bowlders 

 large enough to act as a counterpoise to the shaft he exposes to the force of 

 the wind. 



The high mountain region, the forest region, and the valley region 

 represent fairly three degrees of comparative difficulty in reading the 

 geological story. In the former, except where covered b}- talus slopes at 

 the foot of great clifis, the rock surfaces are all laid Ijare and the geological 

 structure is an open book, only needing an understanding and careful 

 observer to be read correctly. In the forest region there is more or less 

 accumulation of soil and decaying vegetable matter, and rock outcrops are 

 often rare and widely spaced. The record has many gaps which time and 

 care are not always sufficient to fill without resorting to hypothesis or 

 analogv. In the larger valleys, however, whose surfaces are covered to 

 unknown depths by gravel and soil, no outcrops are visible, and induction 

 or analogy are the geologist's only resources for determining the structure 

 of the underh'ing rock formations. 



Glacial formations. — In tlic Arkansas Valley, as already noted, there is dis- 

 tinct evidence of the existence of a glacial lake, and the Arkansas Lake 

 beds, composed of stratified sands, marls, and conglomerates, have been 

 actually exposed in a thickness of several hundred feet. In the South Park, 

 on the other hand, no such stratified deposits have been observed, nor is the 

 topography such as to suggest the possibility of a local lake of any great 

 extent having been formed there during the Glacial period. While the 

 existence of such a lake in the South Park is therefore considered improb- 

 able, the fact that the exigencies of this work admitted the examination of 

 only a small portion of its surface, immediately adjoining tiie Mosquito 

 Range, does not justify a positive statement to this effect. 



