294 GEOLOGY AND MINING INDUSTRY OF LEADV1LLB. 



seen to have been deposited unconformahly upon the Cretaceous. But in 

 the district included in this examination no Tertiary beds are found. More- 

 over, it is not impossible that later and more detailed studies may lead to a 

 modification of this view. Secondly, although the older eruptives are only 

 found in Paleozoic formations within the limits of the map, rocks almost 

 identical were observed in Triassic and even in Cretaceous strata, not far be- 

 yond those limits. Moreover, the same class of rock, as will be shown below, 

 is found in other parts of Colorado to cut through the latest Cretaceous beds. 

 It seems probable, therefore, that though the eruption of this type of 

 rock may have commenced much earlier, it lasted in this region till near the 

 close of the Cretaceous. As regards the age of the younger series; the en- 

 tire absence of Tertiary beds in the region renders it impossible to assign their 

 eruption to any particular division of this era. The time that elapsed between 

 the eruption of the last of the older series and the first of the younger must, 

 however, have been much longer than the above statement would seem at 

 first glance to wan-ant, since in it not only were the inclosing beds elevated 

 above the ocean, and by plication and faulting brought practically into their 

 present position, but erosion must have removed their upper portions down 

 to a general level, which could not have been much higher than that of the 

 average peaks and ridges of the present day. 



But little direct evidence was obtained as to the relative age of the 

 varieties composing either group, but what was found, as well as the indi- 

 rect evidence and a certain indefinable habitus of the rocks, goes to con- 

 firm Clarence King's theory 1 that in each series of rocks composing a local 

 eruption, and which may be considered in general to have a common source, 

 the acidic rocks were the earlier, and the more basic followed in the order of 

 their relative basicity. Thus, among the older series of rocks the more basic 

 porphyrite is younger than the acid quartz-porphyries. Direct evidence of 

 this is confined to the single instance observed of actual contact of the two 

 varieties of rock on the extremity of the east spur of Mount Lincoln; and 

 here, owing to the character of the exposure, the apparent cutting of Lin- 

 coln Porphyry by porphyrite cannot be considered entirely unquestionable. 

 The external habit and internal structure of the rocks, however, both con- 



1 Exploration of the Fortieth Parallel, Vol. I, Systematic Geology, p. 715. Washington 



1878. 



