peale.] GEOLOGY ^ORPHYRITIC TRACHYTES. . 167 



with exactness. Between D and D' there are Cretaceous rocks. Treas- 

 ury Mountain forms the center of the second area. The Cretaceous 

 rocks in this region have been lifted up and so broken, that the lines 

 of outcrop cannot be distinctly seen from a distance. For more detailed 

 information in regard to the rocks here, the reader is referred to Mr. 

 Holmes's report. 



E. — Only the western end of this area was visited. A view of it is 

 seen in Plate XI, at b and c. The area E' is seen at d, and E" at e. The 

 two latter are evidently parts of the same dike; a branch of the creek 

 separates them. At E" the trachytic simply caps the ridge, forming 

 a small isolated patch. This dike was once evidently couuected with 

 the trachyte east of station 34 (the area F). The dike d in the illus- 

 tration is evidently connected with the mass b. The rock from the 

 dike differs slightly from that already described, the mica not being so 

 predominant. It also contains more hornblende. It is of a very light 

 gray color, very compact and fine-grained. 



F. — This is the most irregular, and by far the largest of the trachytic 

 areas. It includes 50 or GO square miles. Our most eastern station in 

 this area was station 34. In ascending the ridge towards the station, 

 there was noticed an iudistinct lamination in the trachyte; some of the 

 beds have calcite. There seems to be an alternation of hard and soft 

 layers, rocks from the latter weathering with rounded edges. The trachyte 

 is more vesicular than any yet described ; the crystals of feldspar 

 are clearer aud have more of a glassy appearance. The rock is gener- 

 ally of a dull-gray color. The weathering of the mass, of which station 

 34 is the center, is very different from that of Marcellina, and of the 

 ridge forming the area E. The mountains here are more massive, aud 

 the creeks draining them head in amphitheaters, separated from each 

 other by long sloping spurs which have very steep ends. To the nortk- 

 east they descend to the top of a mesa of sandstones. This mesa on 

 the opposite side of the creek is shown in the forgeround of the picture 

 in Plate XL 



On the east the descent to the level of the creek is very steep. There 

 are remnants of Cretaceous shales, with coal still resting against the 

 ends of the spurs. 



The arm that extends to the southward, west of station 36, rests 

 against a mass of breccia, which will be described with the next division 

 of the eruptive rocks. The northern edge of this breccia is indicated 

 by a dotted line. In this southern area there are four or five prominent 

 peaks, none of which we visited. 



Between the southern arm and the southern prolongation of the 

 western arm around station 37 is an irregular area of Cretaceous, which 

 has been fully described in a previous chapter. The eruption of the 

 trachyte seems to have caught these rocks and tipped them up in all 

 directions. They seem also to have been much metamorphosed. 



Iu the western arm, stations 38 and 39, are the principal peaks. In 

 the arm connecting this part of the area with that to the eastward there 

 are several peaks equally high. Station 38, as viewed from the west, 

 is a sharp, conical peak, rising very steeply for 1,800 or 2,000 feet, and 

 from that poiut sloping gradually towards Smith's Fork. Fig. 1, Plate 

 VII, gives a section from the station westward. Station 39 is also sharp, 

 and also shows very prominently from the north. The rock of both 

 mountains is the same. Numerous white crystals of feldspar and a few 

 crystals of hornblende are imbedded in a gray matrix. The country 

 around both stations is comparatively low and filled with soft shales of 

 the Upper Cretaceous layers which form low butte-like hills. Seen from 



