212 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



Frotoginyte. — In addition to the granites proper, a variety very nearly 

 related to it was observed in Colorado. At the head of Chalk Creek a 

 very interesting rock was found to compose several high peaks of the 

 Sawatch Eange.* In my report upon that section I have regarded it 

 as porphyritic protoginyte. It occupies a circumscribed area with the 

 granitic region, and may, perhaps, be regarded as a granite with the 

 substitution of mica by chlorite. In all its general features it closely 

 resembles the granite. I consider it expedient, however, to mention it 

 here, as one of those rocks presenting typically the character of an erup- 

 tive. In age I consider it synchronous, or very nearly so, with the 

 granite of the Sangre de Cristo. A crystalline aggregate of orthoclase, 

 oligoclase, quartz, chlorite, and hornblende constitutes the rock. Ortho- 

 clase occurs in Carlsbad twins, inclosing laminae of white, transparent 

 oligoclase. Crystals of the latter are dispersed throughout the rock. 

 Quartz is gray and transparent ; it is found, partly, in fragmentary 

 pieces, partly in grains. Chlorite substitutes the mica of granites, is 

 green to black, and occurs in fragments and small crystals. 



* Eep. U. S. Geol. Surv., 1873, p. 337. 



