GRANITES OF TIKES TEAK. 471 



The next })aper was read, in the absence of the antlior, by F. D. Adams : 



SOME DIKES CONTAINING "IIURONITE'' 

 BY ALFRED E. BARLOW 



The followins^ paper was presented by the author, who was intro- 

 duced to tlie Society by W. B. Clark : 



THE GRANITES OF PIKES PEAK, COLORADO 

 BY EDWARD B. MATHEWS 



Contents 



Page 



Aim of the paper 471 



Area of occurrence, ago and composition of the granites 471 



Types 472 



Number and grouping 472 



Pikes Peak type 472 



Summit type 472 



Cripple Creek type 472 



Fine grained type 472 



Genetic sequence 472 



Conclusions 473 



Aim of the Paper 



It is the aim of this paper to empliasize and corroborate the views lield that a 

 large magma may show variations in structure and composition, and that tliese 

 variations may be made evident Ijy a series of ahnost contemporaneous erui)tions of 

 matter each part of which cuts tli rough the previously formed portions. 



Tlie particular feiiture in this general aim is to give an illustration of such a vary- 

 ing mass in whicli all of the facies belong to the same closely defined rock-type, all 

 of the variations being in the size of the grain and tlie character of the texture. 



Area of Occurrence, Age and Composition of the Granites 



Tlie rocks which form the basis of this discussion are granites from the IMkes 

 Pciik (hstrict, collected during the last two ( lS9.'i-'94) field seasons. The area covered 

 is a little over 900 S(iiiare miles, situated at and including the **en echelon " ending 

 of the Rampart or Colorado range, along the divide between the Platte and the 

 Arkansas rivers. 



The age of these granites is Algonkian, as is evident from their included masses 

 of schist and (juartzites, whose sedimentiiry origin will ))ediscuss(;d in u f<jrthcoiiiing 

 reiK)rt by Whitman Cross. 



Mineralogically they are invariably composi'd of (luartz, microcline and biotite, 

 to which are a(Me<l occasionally apatite, zircon, fluorite, hornblende, epidote and 

 LXVII — Hull. drm.. Soc. Am., Vol. r,, 1894. 



