Stone — Granitic Breccias of the Cripple Creek Region. 21 



Art. IV. — The Granitic Breccias of the Cripple Creek 

 Region ; by George H. Stone. 



In Lis report on the geology of Cripple Creek* Mr. Whit- 

 man Cross has devoted considerable space to the fragmental 

 volcanic rocks of the district. In the present paper I wish to 

 record some additional facts as to the dike phenomena of the 

 region in question, the result of observations made in the years 

 1*94-97. The following named dikes include only the more 

 typical localities. They have been purposely selected from 

 outside the central volcanic district, where they are in the 

 midst of granites and the older cystalline rocks. In such situa- 

 tions the dikes are isolated and it is easy to distinguish their 

 deposits from other volcanic matter. 



Tuffs and breccias composed chiefly of volcanic rock, some- 

 times with an admixture of older rocks, have long been known. 

 They are well represented in the Cripple Creek district, and 

 having been well described by the authors above cited, need 

 not here be further mentioned. The fragmental rocks now 

 to be described are remarkable for consisting of non-vol- 

 canic rocks. They are of a kind not before recognized, within 

 my knowledge, as of volcanic origin. Their general character- 

 istics are the following. 



They are invariably found capping and bordering the out- 

 crop of volcanic dikes, and in no other situations. My conclu- 

 sion is that this relationship is causal. They consist of frag- 

 ments of granites, schists, and others of the older rocks that 

 are penetrated by the dikes. The fragments composing them 

 vary in size from rock flour up to four feet or more in diame- 

 ter, and all are more or less smoothed and rounded, often as 

 much so as the stones on the ocean shore. When the granitic 

 fragments are uncemented they are found at the sides of the 

 dikes, where they overlie uneven surfaces of the granites and 

 schists, proving that the country was already much dissected by 

 erosion at the time of their deposition, and the same is true of 

 all the granitic breccias ; and in this case, if there is a breccia 

 of volcanic rock accompanying the granitic gravel, it overlies 

 the latter. When the granitic stones are cemented into a Arm 

 grit the positions are reversed ; the upper portion of the grit 

 consists wholly or almost wholly of granitic fragments, but as 

 we go down in shafts we find an increasing amount of the 

 volcanic rock mixed with the granitic, until we reach a dike or 



* Geology and Mining Industries of the Cripple Creek District, Colorado. By 

 Whitman Cross and R. A. F. Penrose, Jr. Extract from the Sixteenth Annual 

 Report of the U. S. Geologial Survey, 1895. See pp. 50, 60-64, 78, 81, 86. 88, 

 90, 92, 94, 95, 100. 



