154 



erally shows lighter colors. Near station 21 the changes, as we go 

 higher, can be admirably studied. At a number of points a coarse 

 conglomerate appears in the upper portion of this group, but 1 had 

 no opportunity to study it carefully. Higher up a series of strata occurs, 

 altogether from 400 to 600 feet in thickness, which generally show little 

 but a compact paste, without any segregated minerals. Probably this 

 character is due to having been reheated by the superincumbent vol- 

 cauics. On some of the higher peaks, near the Eio Grande, this series 

 is well developed, but may be regarded as a local pljenomenon. 



By far more varied than the three preceding divisions is E"o. 4, the 

 stratum that reaches its best development in and northeast of the 

 Baker's Park region. Within quite inconsiderable horizontal and vertical 

 distances the rocks of this group show great variation in their lithologi- 

 cal character. Oligoclase and another triclinic feldspar, that may be 

 andesite, generally replace sanidite, although the latter is not entirely 

 wanting. Mica is almost entirely wanting. As a rule the stratifica- 

 tion in the higher members of No. 4 is very distinct, less so in the lower 

 ones. On Handle's Peak, station 14, near the headwaters of Lake 

 Eork, this formation is particularly rich in varieties. As a rule the 

 paste is of a dark bluish to maroon color, sometimes with a greenish 

 tinge, while the feldspars contained in it are yellowish. Small crystal- 

 line fragments of compact feldspar are frequent in the higher strata of 

 this peak, and it is they that usually impart the greenish color to the 

 rock. They receive their color in turn from a percentage of pro- 

 toxide of iron. Near the summit is a band of whitish rock, ^bout 200 

 feet in thickness, that appears like a matrix without any segregated 

 minerals. Descending in a vertical line, the changes of texture are 

 more apparent still, until the "red stratum" is reached, which serves 

 as a very welcome geognostic horizon. It is an aggregation of feld- 

 spathic material, containing innumerable minute crystals of pyrite, at 

 some localities crystallized sanidite ; at others, small crystals of horn- 

 blende. When perfectly fresh, it is white, but soon turns yellow and 

 red. Of the rocks containing the lodes I shall speak hereafter. Above 

 these four groups we find rhyolite and basalt at some points. 



Traveling up the Rio Grande, toward Cunningham Pass, a promi- 

 nent red peak may be noticed on the north side of the river, about three 

 miles west of Pole Creek. The red color appears to have originated from 

 one stratum, and is gradually lost toward the base. Near the head- 

 waters of Lake Fork, station 12 is located. To the northwest of this 

 station a high ridge extends for some distance, remarkable for its bril- 

 liant red color. All along Mineral Creek, which flows into the Eio An- 

 imas at Baker's Park, the same colors can be observed. An extension of 

 this is found on the group of mountains on which stations 27 and 28 

 were made, and near the edge of the mountains toward station 34. 



It is a well-known fact that pyrite, upon decomposition, imparts to 

 rocks and the sediments of water a color identical with the one ob- 

 served in these four localities, and it seemed highly probable, therefore, 

 that this too was owing to the same cause. Inasmuch as the decompo- 

 sition of this mineral progresses rather rapidly, it may be difficult to 

 obtain, without much work, the proof of the supposition. On Mineral 

 Creek, however, specimens were found that were a grayish white, con- 

 taining in part perfectly sound pyrite, in part the same mineral having 

 undergone more or less decomposition. In the latter case, the very mi- 

 nute crystals were surrounded by a brown or red stain, produced by the 

 secondarily-formed hydrated sesquioxide of iron. At another point, 

 station 15, east of the' headwaters of the Eio Animas, it was observed 



