CHAPTER IV. 



AGE, COMPARISON, AND ORIGIN OF ERUPTIYES. 



AGE OF THE ERUPTIVES OF COLORADO. 



Taking a general view of the subject, it is a comparatively easy mat- 

 ter to arrive at conclusions regarding the age of eruptives in Colorado. 

 The more narrowly we attempt to draw the lines, however, the less ma- 

 terial for accurate definition do we find. Though it is not difficult to 

 assign to eruptives a period embracing the same amount of time that 

 is allotted to some particular formation, it is less easy to state within 

 precisely what portion of the period the greatest eruptive activity fell. 



Arranging the plutonic eruptives chronologically, we have — 



Biorite, oldest. 



Euphoiide. ' 



Porphyry. 



Granite. 



Protoginyte. 



Granite. 



Granite is mentioned twice, as we find two distinct granite groups in 

 Colorado. 



With reference to diorite, euphotide, and porphyry we may say that, 

 essentially, they stand in no direct relation with sedimentary forma- 

 tions. In the case of porphyry we have one instance in the Sangre 

 de Cristo Mountains, where it has broken through Carboniferous strata. 

 With this exception we may consider these rocks as pertaining more 

 particularly to the metamorphic series. We find dikes and large erup- 

 tions of them within the metamorphic areas. Fortunately they are 

 usually so placed that no mistake can be made as to their identity, which 

 otherwise might become a matter of serious difficulty. It may be stated 

 that granites, probably of plutonic eruptive character, are associated at 

 times with euphotides. 



It seems very difficult to assign any definite age to the rocks of these 

 groups. So far as can be determined from association with sedimentary 

 groups, their age may be regarded as Post Carboniferous. All evidence 

 in Colorado points to the tact that only unimportant strati graphical 

 changes took place before the close of the Carboniferous or even Tri- 

 assic period. From this it may be inferred that the anarhactic and 

 other eruptives, that we must regard as the oldest in Colorado, were 

 synchronous with the first main disturbances. In view of these facts 

 we can, perhaps, most correctly state that they must be considered as 

 Post-Carboniferous and Pre-Cretaceous. 



The older granites and protoginytes occupy prominent positions, and 

 from their associations afford some indications as to their absolute age. 

 We have observed that mainly Carboniferous and older beds were af- 

 fected by the upheaval of the northern Sangre de Cristo Range. Evi- 

 dently the rise then effected was sufficiently great to exclude the trans- 

 mission of Cretaceous waters directly westward. A possibility presents 

 Itself that at the period of Cretaceous invasion the more easterly por- 

 252 



