PEALE.] GEOLOGY BASALT EAGLE KIVER. 171 



I have already stated tliat tbe origin of this trachyte and breccia is 

 in Dr. Endlich's district, south of the Gimuison. Since it was poured 

 out, the mass of mountains described under the head of Porphyritic 

 Trachytes have been thrown up, which fact accounts for the abrupt 

 bending of the trachyte northeast of station 79, and the general slope 

 toward the Gunnison. The subsequent erosion has been sufficient to 

 remove the trachyte around station 31, and on the ridges running 

 southward from tliis mass of mountains. The amount of the denuda- 

 tion on the Gunnison is measured by the distance between the top of 

 the mesas and the level of the Eiver. 



BASALTIC A-REAS. 



The basaltic rocks of the district all closely resemble each oth^. 

 They are generally dark-colored, gray to black, and are fine-textured. 

 They contain olivine, sometimes free quartz, the latter not abundantly, 

 and on being pulverized magnetic iron can be extracted from them. 

 The latter always caused a great deflection in the needle at all stations 

 made on these areas. Vesicular varieties occur in many places. The 

 different varieties, however, will be described as we proceed. The short- 

 ness of the time at our disposal in the preparation of this report pre- 

 cludes the possibility of giving definite analyses of the rocks. 



As already stated, the basaltic rocks are confined to the northern 

 part of the district, where they generally form the capping of plateaus 

 or mesas, showing that they are lava-flows. Their source was probably 

 to the northward, as, with the exception of ooe locality, I could find no 

 evidence of their having originated within the limits of our region. 



As the general features of the couotry have been already given io 

 considerable detail 1 will confine myself mainly to the description of the 

 rocks and their mode of occurrence. 



Eagle River. — On the summit of the ridge southwest of the Eagle 

 Eiver, opposite the second caiion, there is an isolated area of volcanic 

 rock which I call basalt, although I could discover no olivine. Its ap- 

 r)earance, however, closely resembles that of the rocks in the same re- 

 gion that are undoubtedly basaltic. It is dark bluish-gray, rather com- 

 pact, with a slight tendency in places to lamination. There are a few 

 points of free quartz and numerous yellow spots of some decomposing 

 mineral, which may be olivine. The area occupied by this rock is lim- 

 ited, comprehending only about nine square miles. It is shown on map 

 A. A section across it is shown in Fig. 1, Plate I (/ to g). It will be 

 seen that it rests on the upturned edges of the Eed Beds, and in places 

 touches the Jurrassic or Cretaceous layers. The section in the figure is 

 partly ideal, but I think it presents the true relations of the rocks. 

 It seems that tliere must have been a fissure through which the mate- 

 rial was pushed, and afterward spreading out, it covered the edges of the 

 strata tipped up by the same force that caused the flow. If it is simply 

 a flow that has spread over the upturned edges of the strata it must 

 have come from the north. The hills north of Eagle Eiver shown at c c, 

 in Plate II, are capped with volcanic rock. It is not probable, however, 

 that this rock is the same flow, although it may be of nearly the same 

 age. We cannot be exact as to the age, although it is probably com: 

 paratively modern. All we can say positively is that it is Post Creta- 

 ceous at this point. I think this flow and the one near the mouth of Eagle 

 Eiver were contemporaneous. The latter, as I shall show, is of recent 

 date. It is near the mouth of the river on the north side, and, although 

 outside of the limits of our district, I wish to refer to it. A more de- 

 tailed description will be given by Mr. Marvine. The flow had its 

 source in the hills on the north side. It came down the ravines and 



