G. H. Stone — Granitic Breccias of Colorado. 185 



small fragments of schists with some granite. In several 

 places this is shown by mining tunnels to be fifty and some- 

 times even more than one hundred feet thick. I found one 

 tunnel that had penetrated the breccia into granite and schists 

 in place, in the midst of which is a vertical dike of quartz 

 porphyry bordered on each side by a vertical layer of breccia 

 several feet in thickness. This was composed almost if not 

 entirely of crushed granite and schists. In places the granitic 

 breccia forms a terrace along the side of the mountain. I 

 noticed one place where this breccia rests on schists that have 

 been broken, faulted, and brecciated along the lines of fracture, 

 so that blocks of schistose rocks twenty to sixty feet in diame- 

 ter are separated by zones or vein-like bodies of schist-breccia 

 one to four feet thick. The fragments are only a little rounded, 

 although different kinds of schists are promiscuously mixed, 

 showing that there was considerable faulting. 



In a paper on the "Granitic Breccias of the Cripple Creek 

 District" * I have described somewhat similar breccias to those 

 found in the Grizzly Peak district, but on a far larger scale 

 than the latter. However, the latter are very suggestive when 

 we study the question of origin. 



Obviously when ejectamenta were hurled by violent explo- 

 sions over a large area adjacent to the volcanic vents, to form 

 substantially horizontally-bedded breccias or conglomerates, as 

 was the case in the San Juan mountains, Colorado, and in the 

 Sierra Blanca range, Lincoln County, New Mexico, the frag- 

 ments of the older rocks that were ejected from the vents 

 ought to have been covered and mixed with later ejectamenta 

 composed of volcanic rocks. But in the case of the dikes at 

 Cripple Creek above cited, also those at Grizzly Peak, we find 

 the fragments of older rocks overlying the fragments of vol- 

 canic rocks, and these in turn overlying rather narrow dikes 

 that show but little lateral flow of the lava. The interpreta- 

 tion is plain. These dikes rose quietly and slowly through the 

 pre-existing rocks. Before reaching the surface they became 

 sufficiently solid at their upper extremities to be able to push 

 fragments of the older rocks before them. In some cases the 

 latter were at once cemented fast to the tops of the rising 

 dikes or to the fragments of volcanic rock that were formed 

 by the breaking up of the thin crust of solidified lava, as the 

 dike slowly but irresistibly rose above the pre-existing surface. 

 In such cases they were cemented fast to the rising dike before 

 they could be washed away. In other cases, as the dikes rose 

 above the original surface, the fragments of granite, schists, or 

 other rocks penetrated by the dikes, remained for a time unce- 

 mented. Under the action of gravity, frost, rain-wash, etc., 



*This Journal, vol. v, Jan., 1898. 



