PEALE ON ERUPTIVE MOUNTAINS IN COLORADO. 553 



between them and some of the areas to be described is only in degree. 

 They are fully described in Mr. Marvine's report for 1873.* 



The first of these areas is in the vall<;y of Bine River, on the east side, 

 about live miles south of the Grand. Here at one point the hills of Cre- 

 taceous shales are covered with a large mass of eruptive rock. South- 

 west of this are two dikes of light gray or greenishwliite porphyritic 

 trachyte, which intersect each other at tlieir southern end. One of these 

 dikes is vertical, toward which the other inclines, probably connecting 

 with it below the surface. Mr. Marvine says : — " The trend of these two 

 dikes is toward a hill which lies about eleven miles from the Grand, 

 and which, in its isolation ami abruptness, presents a unique topograph- 

 ical feature on the otherwise regularly formed valley, and therefore in. 

 dicates some equally unique geological fact".t This hill is composed of 

 massive beds of trachyte separated by layers of soft Cretaceous shales. 

 The "intrusive masses" of trachyte, Mr. Marvine says, are "of Post-Cre- 

 taceous age, which, instead of breaking across the strata, here followed 

 along their planes of bedding, and forcing apart and upward the strata 

 between which they wedged themselves, caused them to incline east- 

 ward at a steeper angle than those on either side".| In one place, three 

 of the beds of trachyte are united in one, and near this is what appears 

 to be the side or edge of a flow, against which the undisturbed slates 

 abut on the south, dipping 10° east. From this point north, the trachyte 

 becomes a layer, and resting on it are the slates that have been turned 

 up by it. The beds included between the trachyte layers gradually 

 increase their dip in ascending the hill. 



On the west side of Blue River is a massive hill, also apparently of 

 trachyte, a lemnant of the thickening dike, with the capping slates 

 eroded away. The rock throughout is a handsome porphyritic trachyte 

 with a tendency to very large feldspar crystals. The lower bed of 

 trachyte rests on the sandstones of the Dakota group (No. 1 Cretaceous). 

 The sedimentary rocks seem to have been but little affected by any heat 

 that may have accompanied the eruption. 



Parl< View Mountain is in thedivi<le between Middle and North Parks. 

 From the description given of the rock, and a certain resemblance to the 

 Spanish Peaks, I have included it in the eruptive class. Mr. Marvine 

 says of it and an adjacent hill that they are composed of horizontal 

 Lignitic rocks, with dikes'of a handsome porphyritic trachyte. • Where 

 several dikes intersect or occur near one another, their combined re- 

 sistance to erosion has formed a hill, every s[)ur of which contains a 

 dike. The rock is a grayish-green paste, with numerous large well- 

 formed crystals of white orthoclase and short hexagonal crystals of a 

 soft dark green chlorite. In physical appearance, it resembles the rock 

 of ^yest Spanish Peak. 



'Annual Report Uuitwl States Geological Snrvey for 1873, 1874, pp. 174, 185, IdO. 

 tAnuual Report United States Geolot,ncal Survey for 1873, 1874, p. 186. 

 t Ihid. 



