ENDLicH.] CONCLUSION GRANITE SILURIAN. 105 



Stratification has been retained in a measure, and were it not for the 

 fact the quartzites there appear to be even older than the contiguous 

 gneisses, schists, and granites, they could readily be regarded as merely 

 changed by some locally-acting force, and not by that which is desig- 

 nated at catogeneous. In all its relations this quartzite agrees closely 

 with the groups above enumerated. It shows varieties, dependent upon 

 admixtures of accessory minerals, and has preserved, though in a changed 

 form, the characteristic of a sedimentary group. 



Granite may be regarded as the best represented species of this group. 

 It is both the most widely extended and the one showing the greatest 

 number of variations. Its usual composition, feldspar, quartz, and 

 mica, is changed proportionally by predominance of one or the other 

 mineral, and thus are distinct varieties produced. Accessory minerals, 

 such as hornblende, tremolite, tourmaline, zircon, magnetite, and others 

 impart to it local characteristics that will be found to be constant 

 within certain limits. Bands or zones, almost strata, of specifically 

 different granite may be seen in definite horizons within the heavy, 

 bulky masses composing a range or chain system. These, more par- 

 ticularly, furnish accei»tablc evidence of the metamorphic character of 

 the rocks under observation. Interbedded, if the term can be used, are 

 not unfrequently gneisses and schists, the latter showing a number of 

 varieties. In the districts which I examined, the metamorphic granites 

 form a very prominent feature. Younger, generally, than the schistose 

 rocks occurring near or with them, they form high, steep ranges or 

 series of smaller ones connected by the same material. In the Quartz- 

 ite Mountains their genesis could most favorably be studied. There 

 the direct transition from sedimentary beds into typical granite 

 was observed. In case a detail survey could be made of that locality, 

 I am persuaded that the very beds, unchanged and metamorphosed, 

 could be identified. So far as my observations extend, I am inclined 

 to regard a very great portion of granites in Southern Colorado as meta- 

 morphosed Silurian, Devonian, and, in rare instances, even Carboniferous 

 strata. These formations have furni!?hed the material which, by plu- 

 tonic activity, has been transformed into the condition in which we now 

 find it. No doubt much of the pre-Silurian material has been subjected 

 to a treatment attaining the same end, but we have direct evidence 

 pointing to the fact that younger formations also have been utilized. 

 It is not the purport of this paper to enter into details regarding miu- 

 eralogical coostitution of the various granite^ observed. Keferences 

 thereto will be found in the Annual Reports of the Survey. Gradual 

 or more rapid cooling of the metamorphosed masses has resulted in the 

 production of many varieties. All of these, however, can be recognized 

 as belonging to one great system. 



Metamori)hics generally form prominent topographical features, easily 

 recognizable, and, as a rule, determining the character of the country. 

 Their behavior with reference to erosive agents has been discussed in 

 Chapter III of this report. 



SILURIAN. 



In various parts of Colorado, Silurian beds have been observed. 

 They crop out, at lengthy intervals, along the Front Eange. In the 

 interior of the State the Silurian formatioQ reaches a greater develop- 

 ment. Within the districts I have examined, I found it in 1873 and 1874. 

 The first observed was in the vicinity of the Arkansas River, both north 

 and south of it. Again, it was seen near the Animas River, in Southern 



