ABSTRACTS AND DISCUSSIONS OF PAPERS 399 



has been found showing that the granite is post-Algonkian, and probably post- 

 Peunsylvaniau, in age. 



The probable Mesozoic age of the granite simplifies the interpretation of 

 the structure of the Coal Creek quartzite and does away with the necessity 

 of faulting to explain the relation of the igneous to the sedimentary rocks in 

 the vicinity of the South Boulder peaks. 



The lithology and structural relations of the granite suggest a thick covering 

 of sediments at the time of its intrusion, which throws additional light on 

 this part of the front range as a positive block in the earth's crust. 



Presented in full extemporaneously. 



Discussion 



Prof. Erasmus Haworth asked if there are fragments of granite in quart- 

 zite. 



Mr. Schneider replied that the granite is not gneissoid ; that it shows no 

 indication of crushing, and the quartzite pebbles are mashed and elongated. 



Further remarks were made by Prof. H. B. Patton and Dr. E. S. 

 Bastin. 



OCCURRENCE OF FLOW-BRECCIAS IN COLORADO 



BY HORACE B. PATTON 



{Al)St7'aCt) 



That igneous magmas intruded through and into the overlying rock forma- 

 tions are prone to pick up and incorporate fragments of all kinds and sizes 

 on their way to and over the surface is, of course, no recently recognized 

 phenomenon. The occurrence of such foreign inclosures has long been recog- 

 nized as one of the common features of volcanic action. It would seem, how- 

 ever, that sufficient emphasis has not been laid on this subject by most investi- 

 gators of igneous phenomena, and that the extent and frequency with which 

 such rocks occur has not met with wide recognition. 



In his investigation of the very extensive volcanic series of the San Juan 

 Mountains in Colorado, Whitman Cross some twenty years ago observed some 

 striking instances of igneous rocks in the form of dikes and sheets picking 

 up fragments of the country rocks through which they broke their way to the 

 surface and was the first to apply to such occurrences the name flow-breccia.' 

 In his description of the rhyolites of the Intermediate Series and of the Potosi 

 Rhyolite Series of the Telluride quadrangle. Cross describes in some detail the 

 occurrence of these flow-breccias and finds them often difficult to distinguish 

 in the field from true breccias and tuffs. For instance, of the rhyolites of the 

 Intermediate Series he says that they consist in part of "apparent tuffs of 

 rather indistinct character, many of which wore found to be flow-breccia — 

 that is, a rhyolite flow lu)lding so many fi'Mgnients of andesite that tlie fluidal 

 matrix becomes (juite inconspicuous and can not be seen with the naked eye. 



MVhltman Cross: I'. S. Oc<)li>Ki<"il Survey Vo\U> No. 57. 1800. 



