216 REPORT UNITED STATES GEOLOGICAL SURVEY. 



1874. Among the most prominent are tbe deposits soutli of Black 

 Mountains, east of the junction of Coochetopa and Tomichi Creeks, on 

 White Eartb Creek, and along the liio Grande. Perhaps the most ex- 

 tensive mass of this material occurs on Wljite Earth, and as the name 

 is at the same time a descriptive one, I shall term the trachyte i^o. 1 

 the White Earth group. 



An average thickness of about 800 -feet may be accepted for this 

 grou[) throughout the San Juan country. It is readily recognized by 

 the striking colors it exhibits. Most frequently white, gray, and yel- 

 lowish colors predominate, but pink, green, brown, and av&n black are 

 not wanting. Upon first glance the tuffs composing this group represent 

 variegated marls. More or less frequently hard interstrata are contained 

 therein, in that case producing terraces upon weathering. Generally, 

 the material composing trachyte JSTo. 1, is a light-colored feldspathic ag- 

 gregate, loosely cemented. At some localities bowlders and fragments 

 are found inclosed, giving the beds the appearance of a breccia. Min- 

 ute crystals of sanidite, hornblende, magnetite, obsidian, and some- 

 times oligoclase are found in the tuffs. On account of their readily- 

 yielding physical constitution they are rapidly attacked and worn away 

 by erosive agents. Water courses, wind, and otlier causes carve out 

 most peculiar shapes from the gradually disappearing soft bluffs. Hard 

 interstrata may give rise to the formation of " monuments." Toward 

 the top of the series not infrequently tliin beds of a hard, brown tra- 

 chyte are interbedded into the tuffs. Succeeding flows of lava have 

 thoroughly baked them and they now are hard, brittle, almost jaspery 

 in their appearance. Here, too, we may often fipd concretions and 

 nodules of various colored jasper and of semi-opal. These two minerals 

 cannot be regarded as characteristic of the White Earth group only, as 

 they occur frequently higher up. 



About three miles south of Black Mountain these tuffs were found, 

 resting upon the andesite of that region. The tuff' here shows a thick- 

 ness of about 200 feet, is white, yellow to pink, greenish, and even 

 brown. Several ravines have been washed into the loosely-cemented 

 material, and their sides are studded with a variety of grotesque forms, 

 the result of erosion. Toward the base tbe tuff becomes more comi)act, 

 is gray, and closely resembles a very coarse-grained sandstone. True 

 to the rule, this is only a local deposit. Soon the tuffs thin out toward 

 the edges, and either disappear altogether, or are covered by younger 

 rhyolitic eruptions. From a section taken in the course of north 80^ 

 east,* the relative position of the tuffs may be seen. No doubt the 

 eruption which gave rise to the formation of Black Mountain occurred 

 not from it, if not the main point of outflow was direcrly there. From 

 there the lava flowed in a southerly direction. This same course was 

 also taken by the tuffs. Outside of the main volcanic group the tufts of 

 this period do not often appear. They seem to be replaced at some 

 points by breccias, but these do not bear the same relations to older and 

 younger volcanics as the tufts. In Wet Mountain Valley a few occur- 

 rences of tuffs were noticed, but they are of no importance. 



Near the junction of the Cochetopa and Tomichi Creeks a compara- 

 tively large mass of andesitic tuffs belonging to this group are found.t 

 White and yellow tufts cover Middle Cretaceous shales and are in turn 

 covered by sanidinitic trachytes. Erosion has dealt with the tuffs in 

 the usual manner. Here, too, the horizontal extent of the tufts is not 

 very great. They have, no doubt, succeeded the eruptions farther east, 



* Rep. U. S. Geol. Surv., 1873, p. 311. t Rep. U. S. Geol. Surv., 1873, p. 345. 



