152 



turesque groups. White Earth and the Rio Grande region, near Lost 

 Trail Creek, furnish good examples for this section. 



No. 2. Twelve hundred feet of red to brown stratified trachyte, not 

 unfrequentl.y containing interstrata of obsidian and of pitchstone resem- 

 bling obsidian. Sanidite is the predominant one of the segregated min- 

 erals. Small crystals of brown to bla'ck mica occur dispersed through- 

 out. 



E'o. 3. Two thousand to two thousand five hundred feet of red tO' 

 brown trachyte, separated readily into No. 3 lower and No. 3 upper. 

 The former has a more or less laminated appearance, and weathers light 

 brown to gray, while the upper is more uniform in texture. 



No. 4. Three thousand to four thousand feet of trachyte, by far the 

 most variable of all in its distinctive lithological characteristics. In 

 color it varies from a dark purplish blue to maroon, frequently weath- 

 ering brown on the surface. As a rule it is rather compact, having a 

 microcrystalline paste, and but few segregated minerals. The lower por- 

 tion of No. 4 contains what I am accustomed to call the " Bed iStratmn,^^ 

 which will frequently be mentioned below. It is of considerable impor- 

 tance in the consideration of the minerals, and presents one of the most 

 striking features in the orographic characteristics of the country. The 

 decomposition of pyrite produces this coloring, and every shade, from 

 white through yellow, grange, and red, to dark brown occurs. These four 

 groups, of an aggregate thickness of 7,000 to 8,000 feet, contain the main 

 mass of the volcanic rocks. Above them follow, but not in such con- 

 tinuous masses nor regular succession, rhyolite, dolerite, and basalt. 



Had not the srratigraphical conditions of these various flows or strata 

 been a very regular one, a recognition of their relations inter se and 

 their respective position could not easily have been obtained. Strange 

 to say, even the points of elevation at which the different groups were 

 found do not vary to any great extent. 



On White Earth, for instance. No. 1 is exposed, giving through the 

 light color of the stratum the creek its name. At that point the eleva- 

 tion is about 9,100 feet, and on the Eio Grande, near Lost Trail Creek, 

 an air-line distance of twenty-four miles from White Earth, the same 

 formation occurs at an elevation of 9,800 feet. Inasmuch as the dip, 

 though gentle, is a north to northeasterly one, this small difference can 

 be accounted for. 



Having recognized the striking uniformity of -the several strata at 

 numerous points, the surprising regularity of the dip and the almost 

 entire absence of later eruptive rocks in any of the lower sections of 

 country point strongly to the common origin from one point of outflow. 

 Taking station 10 and the region immediately north of it as a center, it 

 can be observed that the volcanic strata dip off from it on all sides, ex- 

 cept in the immediate vicinity, where there is some irregularity in the 

 dip. From the neighborhood of that peak the numbers, as distinguished 

 above, can be traced in every direction, and readily recognized. A few 

 miles east of station 10 was the only i3lace where basalt was found in 

 positu in a caSon ; the only place, furthermore, where rhyolite oc- 

 curred in quantity as well as at the station itself. Having two of the 

 youngest volcanic rocks in an apparently so abnormal position, as their 

 dips by no means indicate that they could have flown from some high 

 neighboring point, and taking into consideration the almost undisturbed 

 condition of the surrounding rocks, I have come to the conclusion that 

 the main center of eruption for that section of country was from this 

 point. 



After the eruption had taken place, it seems a subsidence occurred 



