338 GEOLOGICAL SUEVEY OF THE TEEEITOEIES. 



huge amphitlieater, the sides of which slope down steeply toward the cen- 

 ter, a little more gently toward the periphery. Upon first sight, the rock 

 composing this mountain might be called a coarse-grained hornblende- 

 schist; but examination shows that the gray to dark-green mineral is 

 not hornblende, but probably diallage; the white crystalline feldspar oc- 

 curring interstratified with the diallage, is oligoclase; small quantities of a 

 dark grayish-green chlorite are accessories ; also an admixture of mag- 

 netite, containing probably a little chromic iron. In this instance, there- 

 fore, we have a rock that is quite widely distributed in the southern se- 

 ries and northern Italian Alps, characteristic in its structure, although 

 in texture closely related to some others — euphotide. Considerable varia- 

 tion takes place in the texture, owing to changes in the relative quantity of 

 the difierent minerals, and to the introduction of hornblende, whereby a 

 more slaty structure is effected as through mica. The weathered surface 

 is sometimes brown, although more frequently a dark-green, almost 

 black. Station 3 is composed of the same material, and subject to the 

 same variations mentioned above. Besides these points, we find a similar 

 rock farther to the north and a little west of Mount Ouray — high black 

 peaks of pyramidal shape, with clean-cut outlines, presenting faces with 

 steep slopes. Here, however, the rock is more schistose still ; mica occur- 

 ring in more considerable quantity produces this effect. Stratification 

 is also existing there, although the curious peaks in which the granite 

 of station 2 indulges do not occur. At stations 43 and 44 mica occurs 

 in such considerable quantities that the rock might seem more like a. 

 very fine-grained mica-s-chist, with little quartz and radiating hornblende 

 and epidote as accessories. It seems that with these variations it would 

 scarcely be advisable to refer this occurence to the same group to which 

 Mount Ouray belongs ; it seems to be more closely allied with mica- 

 • schists. Upon weathering, the rock scales oft* in single plates or tablets, 

 that cover the sides of the mountain, and by their frequent sliding pre- 

 vent the growth of vegetation. These schists stand in no such intimate 

 connection with the granite as those spoken of above, and the boundary- 

 line between the two is very distinct and sharp. All these points, with 

 the exception of station 3, are located on the continental divide, and rise 

 to considerable elevation. Besides these occurrences, there are no rocks 

 of a similar character in section c except those dioritic ones forming 

 dikes. 



The Silurian formation is found in section c at several points along 

 the base of the granitic mountains bordering the southern end of the 

 Arkansas Yallej^, again surrounding station 38 and near station 42. 

 <jranite foot-hills slope oft" toward the valley of the Arkansas in rounded 

 outlines, and immediately resting upon them we find the heavy Silurian 

 beds, recognizable from their characteristic lithological constitution. 

 Light- colored quartzites of yellowish, bluish, and reddish tints are con- 

 formably stratified with the superincumbent gray to bluish limestone 

 with siliceous segregations, all dipping off uniformly at an angle of 

 about 15° to 80° toward the north. Erosion has formed a number of 

 bluff's along the lower edge of the Silurian rocks; but upward they 

 recline in regular outlines upon the granite. As no fossils were found, 

 I am not prepared to parallelize these formations with any occurring 

 in the Eastern States ; but lithologically they certainly seem to be identi- 

 cal with those found north of the Arkansas Eiver. A section taken 

 through Mount Ouray, running almost due northeast through station 

 55, may illustrate the position. The highest point of station 2 is com- 

 jiosed of granite, (a,) and granite veins traverse the euphotide (&) through- 

 out, which is represented by a series of granitic ramifications occurring 



