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



most of these loose fragments then slid down from the tops of 

 the rising dikes and formed a talus over the lower slopes of the 

 dikes and thence for a short distance over the adjoining country 

 rocks. Here they were subsequently cemented into firm brec- 

 cias. 



I see no possible way for explosive volcanic eruptions to 

 preserve large masses of fragments of the older rocks free 

 from mixture with fragments of lava, and still less to leave 

 them overlying all other volcanic matter. 



Bowlders of the so-called rhyolite of the Grizzly Peak dis- 

 trict are found in the moraines at Twin Lakes. This proves 

 that the volcanic eruption occurred before the glacial epoch. 

 The position and mode of deposition of the granitic breccias 

 prove that at the time of the eruption the Sawatch range was 

 already deeply dissected by erosion, and that there has not 

 been very much erosion since. 



In the paper above cited I have described the gold-bearing 

 granitic sand-rock that formed out of the granite crushed by a 

 rising phonolite dike at the Alhambra mine, Cripple Creek 

 district. The facts there described show that vein waters, 

 probably waters heated by the dike which still remained in a 

 molten condition a few feet below the solidified upper crust, 

 decomposed the grains of crushed granite, honeycombing the 

 sand and replacing much of the feldspar with silica and finally 

 cementing the sand into a firm rock. The mineralized surface 

 crust of sand-rock was in places charged with many times as 

 much gold as is contained in any vein that has been found 

 below in the granite or in the phonolite dike. 



Any complete theory of vein deposits will have to treat of the 

 mineralization of the superficial crusts of breccia that formed 

 above and along the margins of rising dikes. There are numer- 

 ous instances of this class of superficial ore deposits found 

 among the granitic breccias of the Grizzly Peak region, but 

 the subject requires more detailed treatment than is possible 

 within the present paper. 



Colorado Springs. 



