PEALE ON ERUPTIVE MOUNTAINS IN COLORADO. 563 



ditiou in the lower strata similar to that seen in the isolated groups of 

 the southwest, and perhaps, before the removal of the Tertiary in the 

 latter region, we might have found mountains similar to Park View. 



I have noted the fact that, although there is considerable difference 

 among individual specimens of the rocks, there is a general resemblance. 

 Specimens intermediate between the most diverse can be found, forming 

 connecting links. In the descriptions of the areas, the rock is sometimes 

 called trachyte and sometimes granite?, and in the reports of the Survey 

 the names trachyte, rhyolite, and eruptive granite are all used. A jiiece 

 of the granite*? from the Elk Mountains or from Mount Guyot, compared 

 with the trachyte of Middle Park or any of the West Elk groups, or with 

 the feldspathic hornblendic rocks of some of the southwestern areas and 

 the Spanish Peaks, seems utterly different; yet, as I have said, we can 

 find specimens that connect them, and in the Elk Mountains, rocks like 

 those of the other areas can be found occurring near each other and in 

 the same manner. In the larger areas of the Elk Mountains, the rock is 

 compact and homogeneous, resembling a fine-grained granite or syenite. 

 In the smaller areas and in the dikes, the trachy tic character prevails. 

 In the southwest, there is a greater abundance of hornblende, and free 

 quartz does not appear so frequently. Mica is abundant in some 

 specimens, but oftener does not show. Until a chemical and microscopic 

 examination is made, we have, of course, but imperfect data upon which 

 to proceed in treating of the rocks. They are all light-colored, and gen- 

 erally porphyritic. They are feldspathic and acidic rather than basic. 

 The differences may be due partly to different conditions of cooling or 

 different degrees of pressure, or perhaps to differences in the material 

 from which they are derived. Their general resemblance, however, 

 points to some common origin. 



In the Keportsfor 1873 and 18741 was of the opinion that if the trachy- 

 tic dikes could be traced to the junction with the granitic? masses, the 

 two would be found grading into each other ; and in the Report for 1873 

 I referred to the possibility of the Elk Mountain eruptive granite being 

 remelted metamorphic rocks. I am more than ever inclined to this 

 opinion. If these rocks are the result of the fusion of granitic material, 

 we would naturally expect to see differences in them. It is a curious 

 fact that in the San Juan groups, beds of Cretaceous age alone are found 

 included in the igneous mass. It would seem that, in passing through 

 the older rocks, a portion of them at least, would be included in the mass. 

 Mr. Holmes suggests that they have been so included, but have become 

 a part of the igneous material, and are no longer recognizable. In the 

 La Plata group, he assures me that it was very difficult in places to dis- 

 tinguish the igneous rock from the changed shales, where the latter 

 penetrated it. In an almost vertical dike running north from the Car- 

 riso group, the rock is a dark green micaceous rock, closely resembling 

 a micaceous schist, and yet it is igneous. Whether derived from the 

 fusion of metamorphic rocks, or having come from the infra- Archaean 

 3 B H 



