EiDLicH.l AGE OF EEUPTIVES OF COLOEADO. 253 



tions, now covered by beds of that formation, were too low to admit of 

 greater extension of the waters. In this case the subsequent rise of the 

 entire region elevated such strata to their present position. We have, 

 however, in the Sangre de Cristo Eauge tangible evidence in the severe 

 disturbances to which the Carboniferous beds have been subjected. Ev- 

 idence is adduced from various points there observed, that the up- 

 heaval of the granite was in the most intimate connection with the . 

 dislocation of Carboniferous strata. From the nature of these disloca- 

 tions it must be inferred that the strata had already assumed a rigid 

 stability. It can be said, therefore, that the eruption of members of this 

 group must have been subsequent to the last deposition of Carbonifer- 

 ous beds. 



More applicable results can be obtained from the study of the younger 

 eruptive granites. Throughout the Elk Mountains, where they mainly 

 occur, their associations with sedimentary beds are thoroughly constant. 

 They have there been sufficiently well studied to arrive at satisfactory 

 conclusions regarding the period of time when the granitic masses were 

 erupted. In previous pages the contortion, plication, and upturning of 

 Cretaceous strata in and near the Elk Mountains have been alluded to. 

 We have, therefore, in this instance, phenomena whereby to measure the 

 time of eruption. This falls, as nearly as can be determined, after the 

 end of the Cretaceous period, reaching a time either in the Tertiary or 

 after the close of the Cretaceous formation. Such is the accepted view of 

 those who have studied the groups most carefully. There seems to be 

 some connection, not only as to time, but also in composition and certain 

 modes of appearance, between this series of granites and the next group, 

 the trachorheites. Certain connections can be traced iu a number of 

 points between these two classes that bring them very closely together 

 as regards genesis. Different conditions of cooling may produce such 

 results that necessarily the rocks must at present be separated, although 

 their former relations may be very apparent. 



Of greater variety are the volcanic eruptive rocks of Colorado, an 

 enumeration of which will give the following result, beginning with the 

 oldest : 



( Andesite. 

 TracliorJieites : < Trachyte. 

 ( Rhyolite. 

 Porphyritic trachyte. 

 Dolerite. 

 Basalt. 



This series can appropriately be divided into three groups. Of these 

 the first — trachorheites — is the most extensive. In discussing this im- 

 portant group the direct relations to sedimentary strata have been 

 given. On the eastern side of the trachorheitic area no favorable out- 

 crops were observed. Along the western and southwestern edges, how- 

 ever, satisfactory evidence was obtained. We there find the tracho- 

 rheites resting upon Middle Cretaceous shnles. These latter have been 

 removed from their original places of deposition, and the volcanic layers 

 have been spread out over them. This assigns to the entire series an 

 age falling near the close of the Cretaceous epoch. Isolated occurrences 

 show rocks belonging to this group superincumbent upon Post-Creta- 

 ceous lignitic beds. Such relations decrease the age of trachorheites, 

 bringing them essentially wiihin the scope of the Tertiary period. This, 

 too, is the accepted age for the analogous rocks of other countries. 



So far as can be determined the various members of the trachorheitic 

 series form one continuous chronological succession. We can, therefore, 



