BNDLicH.] COMPARISON OF ERUPTIVES. 255 



Volcanic : 



BASIC : ACIDIC : 



fAndesite. 

 Trachyte. 

 ! Trachyte No. 1. 

 Trachoeheites : 1 Trachyte No. 2. f Post-Creiaceoua. 

 j Trachyte No. 3. 



Trachyte No. 4. 

 tRhyolite. 

 Porphyritio trachyte Tertiary ? 

 Symmorphic trachyte. 



Basalt*''' \ ^^*^<^^y ^^^ Post-Tertiarij. 



OOMPARISOl!^ OF ERUPTIVES OF COLORADO WITH THOSE 

 OF OTHER REGIOi^S. 



Although, generally speaking, the occurrences of eruptive rocks in 

 Colorado may lay claim to being unique, they have many points in com- 

 mon with those" of other localities. Any comparison destined to es- 

 tablish similarity of the groups must necessarily fall somewhat short. 

 This is due in part to the fact that technical nomenclature upon this 

 subject is one that will bear very thorough revision and correction. 

 Richthofen has succeeded in establishing definite terms for certain 

 groups of our Western country. In the preceding pages the primary 

 classification as proposed by Bunsen has been used, and subdivisions 

 based upon, primarily, genesis, secondarily chronological succession have 

 been applied. I am of the opinion that a classification of this charac- 

 ter is certainly the most acceptable that could be utilized in the present 

 instance. Though the chronological succession is a method of separa- 

 tion that can lay no direct claim to being one of strictly scientific char- 

 acter, it is the most convenientmannerof distinguishing certain groups. 

 Wherever the evidences of geological age are so perspicuous as in Colo- 

 rado, a system of this kind will answer admirably well. 



We find, upon examination, analogies to nearly all of the erupted rocks 

 observed in Colorado. Far more prominent than in this State are the plu- 

 tonic eruptions of Europe. There the series is well developed, and its 

 members enter very largely into the geological structure of various regions. 

 Diorite, euphotides, and that class of rocks that is comprised under the 

 general name of gabbro, build up high mountains and even mountain- 

 systems. In Colorado their appearance as well as their importance is 

 generally subordinate. Eruptive granites, widely ranging in age, form 

 a prominent feature of study for European geologists. Not only have 

 they broken through sedimentary beds, but they have intruded into the 

 older metam Orphic rocks. Famous among many cases of this kind is 

 that of Heidelberg,* where a finegrained granite occurs, intrusive in a 

 coarse-grained one. By the force of its intrusion the latter has been 

 thoroughly brecciated and shows the altering influences of the younger 

 mass. 



Successively we find in Europe granites that range in age from Silu- 

 rian to Tertiary. It is highly probable that the eruptions of the Elk 

 Mountains vary but little in geological age from those observed in the 

 Tyrol,t where granite has penetrated undoubted Tertiary strata. It may 

 be well to devote here a few words to a subject that, in this connection, 

 becomes one of great importance. We are accustomed to term that 

 rock a granite, for instance, which proves to be a crystalline aggregate 



* Gotta, GeologischeBilder, Leipzig, 1871, p. 195. 

 t Compare Cotta, Geologic der Gegeawart. 



