254 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



accept tbe geological age above given for the entire group. Inasmuch 

 as the ejection of the entire mass must have occupied a very long period 

 of time, it is eminently probable that it commenced within — for in- 

 stance — the Cretaceous und ended during the Tertiary period. We can 

 safely place the age at that period, therefore, synchronous with which 

 we observe the most recent volcanic effects. 



Undoubtedly we must exercise considerable latitude for the chrono- 

 logical limits within which to confine any period of volcanic activity. It 

 is evident that while the eruptions were going on, there was a continued 

 deposition of sediment, which, by its organic remains, may now be re- 

 ferred to either one or the other conventional formation. We cannot, 

 therefore, say definitely at which time the volcanic activity commenced, 

 and we can only say approximately when it ceased. 



The second group, that of the porphyritic trachytes, has been dis- 

 cussed at some length above, and its age has been regarded as having 

 the same most recent limits as the trachorheites. 



The youngest group, that of the basaltoids, is one that forcibly re- 

 minds the observer of lavas of the present day. From all the associa- 

 tions, of both dolerite and basalt, their absolute age can unequivocally 

 be placed within the Tertiary epoch. Within the large areas less can be 

 learned, as a rule, but the anarhactic and some of the isolated eruptives 

 afford definite information. It has been stated in previous pages that 

 the doleritic breccia of Middle Park is associated with beds belonging 

 to the lignitic series, while a number of basaltic outcrops were observed 

 in Tertiary strata belonging to the Eocene group. Frequently the 

 " fresh" appearance of the basalts has been remarked upon. Dr. Peale 

 says,* in speaking of the basalts occurring near Eagle Eiver : "The 

 basaltic rock is destitute of vegetation and comparatively free from soil. 

 It has the appearance of having just been poured out. The period dur- 

 ing which it was poured out is probably to be measured by hundreds of 

 years, and perhaps less, rather than by longer periods." 



From our present knowledge of the basalts of the West, we may say 

 that they effect a transition from the prehistoric to the eruptives of the 

 present day.t Evidences of this fact, far more striking than in Colorado, 

 are observed in adjacent States and Territories. In subsequent pages 

 allusion will be made to such instances. 



With reference to theagesof theseeruptivegroups, as compared among 

 themselves, we may say that the chronological succession is a well- 

 established one. We are enabled, from the extended series of observa- 

 tions that have been made, to correlate the various members and arrive 

 at definite conclusions as to their proper relative positions. An excep- 

 tion to this rule is made by the porphyritic trachytes. Their position in 

 the chronological scale has been discussed previously, and a mere men- 

 tion of the fact may here suffice. 



A classified arrangement of the Colorado eruptives, so far as known 

 and explored at the present time, is here presented. Classiticatiou is 

 based upon the principles given in the introduction of this paper, and 

 their grouping is made with especial reference to the results obtained in 

 Colorado. 



Plutonic : 



BASIC : ACIDIC : 



?l?^}^^;i Ir^ ,r 1 -f §''%°'*-- f \Post-Carhoniferous. 



Eupnotide. > Post-Carhoniferous. Protogiuyte. 5 



Porphyry. ) Granite. I Post-Cretaceous. 



■ Eep. U. S. Geol. Surv , 1874, p. 172. t Compare Eep. U. S. Geol. Surv., p. 222. 



