322 GEOLOGICAL SURVEY OP THE TERRITORIES. 



illustrate this view. From the Cretaceous bay that runs westward past 

 station 11, a small arm of oblong shape extends southward, crossing 

 the Arkausas. The dip all aloug the granite boundary in the bay is 

 toward the east to northeast, Vc-ry slight — only 3 or 4 degrees in the 

 western end of the bay — but the southern arm, which is west of the line 

 of rising, sliows a dip to the west and southwest throughout of about 7 

 to 10 degrees, increasing toward station 11. No merely local cause 

 could be found to explain this ; and as all the evidence obtained points to 

 the fact that the main mass of rhe mountains partook but slightly of 

 the tendency exhibited by the portion nearer to the ancient coast, this 

 fact may be considered as arguing in favor of the adopted view. In 

 the western portion of section a, a gradual rise also took place, which 

 gave the Silurian and Carboniferous their northerly and northeasterly 

 dip. I feel inclined to ascribe this to the eruptive activity going on in 

 the sections farther south, and shall have occasion to speak of it again 

 in discussing the dynamical geology thereof. 



Perhaps contemporaneously with, certainly not very long after, the 

 elevation of the granitic masses, occurred the immense eruptions of 

 trachorheites. In quantity, probably the tuffs and breccias exceed the 

 other forms of volcanic rocks in section a; next are the andesites ; then 

 the trachytes ; and rhyolite seems to be re[>resented by the smallest 

 quantity. The eruptions were mostly massive, not volcanic, although 

 quite numerous cones and craters exist that have preserved their char- 

 acteristic shape very well, and usually present commanding points. A 

 large quantity of the ejected material must have been in a liquid state, 

 while part of it may have been deposited analogous to the deposi»t of 

 ash covering Herculaneum and Pompeii. After " ash "had been thus 

 deposited, frequently subsequent flows of lava covered it, and produced 

 a metamorphic state. The general aspect of the country at the time of 

 these eruptions was probably a very monotonous one ; comparatively, 

 but small effects had been achieved by erosion, as is shown by the very 

 extensive and numerous plateaus formed by the trachorheites. It seems 

 quite likely that the disturbance caused by all these eruptions had a 

 tendency to upheave the main mass of granite, which no doubt occurred ; 

 but it seems further that, in section a, the uplifting of the sedimentary 

 beds of this Front range was not owing to any eruptive action, because 

 in that case local variations from the more general features would.be 

 more frequent and more evident. I may here refer to Plate . Yet 

 a word may be said regarding these sedimentary formations. All along 

 the line of granite, the action was so uniform that almost throughout 

 the same beds rest upon it apparently, with the exception of the two 

 Cretaceous bays. From evidence obtained farther north by Dr. Peale, 

 as well as from a local feature observed in the southern part of section 

 d, it may be conclusively inferred that older formations underlie those 

 that are exposed ; that the Carboniferous and Silurian are not wanting, 

 although they do not outcrop at even a single point throughout the 

 entire Front range of section a. Wherever the rise of the granite has 

 progressed sufiicieutly, the older formations can be and are found. 

 Eeversing the sentence, it will be seen that, wherever they have been 

 identified, the mean relative altitude of tbe underlying granite is greater 

 than the mean of other points. 



The two Cretaceous bays were probably never invaded by Tertiary 

 waters. When the increase of water occurred that we must accept for 

 the Tertiary period of that section, they had already risen too high to 

 Ibe touched by it. 



