208 REPORT UNITED STATES GEOLOGICAL SURVEY. 



rh.yolite are all readily recognized wherever met witb. In 1874 I subdi- 

 vided trachyte into four groups.* Of the basic volcanic products we find 

 only dolerite and baseiU. the latter represented by numerous varieties. 

 A synopsis of the erupted rocks found in Colorado, classified primarily 

 upon chemical, secondarily a genetic, and thirdly upon a chronological 

 basis, beginning with the oldest, will furnish the following result: 



BASIC. ACIDIC. 



Plutonic. 

 Diorite. Granite. 



Euphotide. ^ Protoginyte. 



Porphyry. 



Volcanic. 



Andesite, 

 Trachyte. 



Trachyte, 1. 

 Trachyte, 2. 

 Trachyte, 3. 

 Trachyte, 4. 

 Dolerite. Porphyritic trachyte. 



Basalt. Ehyolite. 



It seems difficult to determine the relative age of porphyritic trachyte 

 as compared with rhyolite. At many localities the one seems to grade 

 into the other. lam not aware that any point was found wbere the 

 two, each typically represented, were in contact. From all analogies I 

 should judge them to have been erupted at very nearly the same time. 

 At one locality, perhaps, a rhyolitic eruption may have been completed 

 before the nearest one of porphyritic trachyte took place, and vice versa. 

 1 place the latter immediately after trachyte, for the reason that it is 

 less acidic than rhyolite, and regard its definite age with reference to this 

 class of rocks as not fully established. Genetically it should be removed 

 from the trachyte and cede its position to rhyolite. 



The classification as above given will represent all the erupted mate- 

 rial found in Colorado. It is difficult to find an acceptable basis for 

 division, but, it seems to me, that for this State, and perhaps others, 

 the arrangement is the most satisfactory. Occurring in connection with 

 these groups are rocks that have frequently been regarded as indepen- 

 dent representatives either of definite kinds of eruptive activity or of 

 characteristic genesis. I allude to such instances where obsidian, per- 

 lite, pumice, &c., have not 'been regarded as minerals merely, but 

 have been designated as rock-species. Their presence or absence 

 certainly is of importance considering the circumstances attendant 

 upon the genesis of the rocks they are associated with. 



CHAPTER II. 

 PLUTONIC ERUPTIVES. 



BASIC PLUTONIC ERUPTIVES. 



Diorite. — Within the metamorphic areas, more particularly of the 

 Front Eange, a number of dikes occur. Traversing the granite in vari- 

 ous directions, we find a hornblendic dike-rock, closely resembling di- 

 orite. " It is composed of small crystalline particles of hornblende and 



* Rep. U. S. Geol. Surv., 1874. 



