GRANITES OP THE ROCKY MOUNTAINS. 75 



grained granite, and otherwise does not describe them as similar to those 

 of the Black Hills.* 



In the survey by Mr. Clarence King of the 40th Parallel no granites 

 were found of the highly feldspathic and coarse character observed in the 

 Black Hills. 



In Colorado the granites and metamorphic rocks of the Middle Park 

 region have been studied with care by Mr. A. R Marvine f He finds them 

 to range from "quartzite through silicious and mica schists to very simple 

 varieties of gneisses and granites, in which the mica is wholly subordinate, 

 and the feldspar mostly a tabular and twinned orthoclase, * * * while 

 the syenitic element and the more basic rocks generally were almost entire- 

 ly wanting." Excepting some minor granitic areas of uncertain eruptive 

 character, Mr. Marvine regards the granites of this region as metamorphosed 

 shale, which over large areas "has reached that last term of metamorphism, 

 viz, structureless granite." 



The intercalation of granite and gneissic strata with a gradual increase 

 of the granite ratio in the direction of the main granitic mass is also men- 

 tioned by Mr. Marvine — an arrangement noticed in the Black Hills. Fre- 

 quent mention is made of the gradual passage from the schists to the 

 granites, of their mutual conformity, and of evidences that seem to prove 

 their metamorphic origin in situ. These granites do not closely resemble 

 those of the Black Hills in composition, and they differ moreover by con- 

 taining many metaliferous fissure veins. Dr. J. J. Stevenson, J who has 

 examined the granites of the southern part of Colorado, finds nowhere the 

 very coarse, feldspathic granite described in the Hills, and from their study 

 is firmly convinced that they are metamorphosed strata. 



Notwithstanding the study that has been given to the subject, the 

 origin of granite is still an unsettled problem. There was a time when 

 it was universally regarded as an igneous or plutonic rock, being at 

 every locality either a part of the original crust of the earth formed 



* Northwest Wyoming, including Yellowstone National Park. Jones, United States Engineers. 

 1873. Geology by T. B. Comstock. p. 103. 



t United States Geological and Geographical Survey of Colorado. Hayden, 1873. p. 139. 

 I United States Geographical Surveys west of the 100th Meridian. Vol. iii, p. 345, ctscq. 



