ESDLicn.] CLASSIFICATION OF ERUPTED ROCKS IN COLORADO. 207 



Great homogeneousness of the component material, and its more regular 

 arrangement with reference to under and over lying beds, facilitate the 

 retention of original distribution of drainage. VVithin the San Juan 

 and Uucompahgre Mountains such features can, perhaps, be best ob- 

 served. There the great horizontal extent as well as the vertical dimen- 

 sions of the volcanic rocks have afforded the most satisfactory oppor- 

 tunities for a typical development of such features. Generally, too, a 

 volcanic region is well watered, owing to the comparative impenetra- 

 bility of the beds. Incident thereupon are promotion of vegetation and 

 formation of soil. Exceptions to this rule are certainly not wanting, 

 leost of all in groups of high, abrupt mountains, and in plains where 

 the absence of water has not permitted vegetation to flourish. 



CLASSIFICATION OF ERUPTED ROCKS IN COLORADO. 



In Colorado by far the greater mass of volcanic material must be as- 

 signed to one geological epoch. This epoch is divisible into two periods. 

 Making, primarily, this distinction, we have made the first step to- 

 ward a classification. Very few cases only were found where erupted 

 material was evidently in no connection whatever with that of the ac- 

 cepted epoch. Isolated occurrences of porphyritic rocks, older, geo- 

 logically speaking, than the remaining eruptives, represent these cases. 



Enumerating the erupted rocks of Colorado, beginning with the old- 

 est, we have granite, porphyry, andesite, trachyte, rhyolite, porphyri- 

 tic trachyte, dolerite, and basalt. Separating these i^rimarily into Bun- 

 sen's divisions, basic and acidic, the result is as follows : 



Basic. Acidic. 



Diorite. Granite. 



Euphotide. Protoginyte. 



Porphyry. Andesite. 



Dolerite. Trachyte. 



Basalt. Porphyritic trachyte, 



Etyolite. 



Of these the diorite, euphotide, porphyry, and granite must be classed, 

 according to Cotta, a^s plutonic, while the remainder are volcanic. These 

 two primary divisions furnish us with the foundation whereon to build. 

 For the western acidic volcanics we have Richthofen's admirable classi- 

 fication. He separates them into propylite, andesite, trachyte, and 

 rhyolite. Four years ago* I comprised all of these under the term 

 Trachorheites. Subsequent explorations proved the existence of a group 

 totally distinct from the trachorheites. It is separated from them by 

 its mode of occurrence, the lithological character of its rocks, and by 

 its relative position. Dr. Pealet has distinguished it under the name 

 of porphyritic trachyte. Trachyte-porphyry of European geologists, 

 their liparitef has a certain resemblance to it, but differs in many essen- 

 tial features. 



So far as could be determined, we have in Colorado no propylite. It 

 is possible that it exists there, or rather its representative, but either 

 its intimate association with younger volcanics or a change in its com- 

 position has rendered it undistinguishable. Andesite, trachyte, and 



* Rep. U. S. Geol. Surv., 1873. t Bulletin U. S. Geol. Surv., Vol. iii. No. 3. 



t Cotta, Geologic der Gegenwort, p. 54. 



