PEALE.] GEOLOGY PORPHYRITIC TRACHYTES. 165 



at au angle of 20° to 25°. It is in these sandstones that the anthracite 

 coal occurs, at the point marked m on the map. On the southern side 

 there are also sandstones, but, as far as I observed, they are horizon- 

 tal. Towards the west end of the ridge, resting on these sandstones, is 

 a mass of breccia, which seems to abut against the trachyte. The line 

 on the map indicates this breccia. A specimen of trachyte from the 

 eastern end of the ridge has a rhyolitic appearance. It is light-gray in 

 color and contains a great deal of free quartz. The matrix is rather 

 coarse, and contains large crystals of glassy feldspar and small hexag- 

 onal crystals of a brownish-black mica and a few small needles of horn- 

 blende. 



Above the coal-bed mentioned as occcurring on the north side of this 

 area there is a layer of trachyte in the shales and sandstones. It 

 resembles the rock just described in appearance and construction, being 

 perhaps a little rougher and having the hornblende in greater quantity. 

 The crystals of feldspar are not so large. The free quartz has the 

 appearance of pebbles, the edges being rounded, probably the result of 

 heat. 



B. — Station 32 is the mountain named Mount Bichard Owen by Lieu- 

 tenant Euffner.* He says, "Mount Eichard Owen has tw^o peaks, the 

 northern of which is the higher, of dusty-red rock, probably ferruginous 

 quartz, or perhajis trachyte." The mass of which station 32 is only a 

 portion, is composed of Cretaceous shales and sandstones intersected 

 and metamorphosed by numerous dikes, of which the principal ones are 

 shown on the map. To accurately define all the dikes in this region 

 will require a very close and detailed survey. Station 32 seems to be 

 a center for them. In-the ridge soutbeast of the station there is a layer 

 of trachyte between layers of sandstone. The section in which this 

 occurs was given in a preceding chapter. This rock is rather more 

 compact and finer textured than the trachytes I have hitherto described. 

 The porphyritic character is not so decided. It is of a light greenish 

 color, and seems to be slightly arenaceous, as though it had taken from 

 the surrounding rocks a portion of its sandy material. It is perfectly 

 conformable with the sedimentary rocks, and, under other circumstances, 

 might be taken for a contemporaneous flow. It presents a square, mass- 

 ive edge on the bluff, as shown at C in Plate X. The dike, marked a h 

 on the map, is also shown on this ridge cutting across it almost at right 

 angles to its strike. It is about 300 feet in width on the ridge and 

 inclines slightly to the northeast. At right angles to it is a narrow 

 seam of quartz, inclining west at an angle of 85"^. This seam could be 

 distinctly traced for several miles on the ridge. I was unable to trace 

 the eastern extension of this dike to its extreme limit. The rock of this 

 dike is a greenish-gray trachyte, rough in texture and containing a 

 large number of feld spathic crystals. 



Crossing the same ridge a little farther north is another dike, the one 

 marked e/on the map. Plate XIII shows the appearance of this dike 

 as seen on the face of the bluff, a a a representing different portions of 

 it intersecting the sandstones hh h h. The rock in this dike differs 

 greatly from the others. It has a very compact, fine-textured, dark- 



_ greenish matrix, in which are a few small crystals of feldspar. 



" They are more conspicuous on the weathered surfaces. Besides the 

 feldspar there are particles of free quartz. The specific gravity of this 

 rock is greater than that of those from the other dikes. The dike c d 

 of the map forms a very prominent ridge leading down from station 32 



* Report of Reconnaissance in the Ute Country, page 4'0. 



