ol8 GEOLOGICAL SURVEY OF THE TEREITORIES. 



eastern slope of the Front range, among which probably the Eocene 

 must be looked for, but our work did not take us near enough to 

 make any satisfactory observations. No unconformability was noticed, 

 although, if there was really one, it would be somewhat obscured by the 

 very small angle at which the strata dip off into the plains. 



This will close the discussion of the sedimentary formations of section 

 a, and only the volcanic rocks and the "removed" material remains. 



South of Pike's Peak, beginning within a short distance of it, we find 

 a large isolated mass of volcanic material, Orographically, this portion 

 of the country is characterized by a number of pointed conical peaks, 

 the recognized type of volcanoes, although it is not to be understood 

 that they are all cones formed by volcanic eruption. Their elevation is 

 generally so high as to let them be commanding points, and several of 

 them Avere for that reason selected as topographical stations^ High 

 plateaus, covered with grass and quaking asp, usually denote the exten- 

 sive flows of lavoid material, and precipitous cliffs, mostly of dark color, 

 frequently show curious freaks of weathering. This one mass seems to 

 be perfectly isolated from all the rest at present, but probably at some 

 former time a connection existed between it and the more westerly por- 

 tions. Going westward, another area covered by it is found after cross- 

 ing Oil Creek. The character is almost identical with that just described, 

 only that the plateaus extend for longer distances. Currant Creek 

 forms the western border of one portion and the eastern of another. 

 Near station 56, the country becomes more rolling, more like high prai- 

 ries, with isolated, prominent peaks of volcanic rock. From there, this 

 material extends northward and spreads considerably. A fourth j)or- 

 tion is located immediately northwest of the heavy mass of Arkansas 

 sandstone spoken of above. Granitic oulicrops occur in several points 

 throughout that portion. To the south it crosses the Arkansas, forms 

 a few low hills, and disappears. The western boundary is nearly 

 parallel with the river-course, following that sharp bend made by the 

 Arkansas about eight miles south of Centreville. Stations 51 and 52 

 are located on a little spur of a volcanic ridge. As a rule, the volcanic 

 rocks of section a overlie granite, but in several instances they cover 

 sedimentary beds. Erosion, perhaps not without the help of some 

 other agent, has carried away portions of them in such a manner as to 

 leave the beds of the largest creeks in granite. Interesting studies 

 could be made by attempting to trace the direction and extent of every 

 flow of these lavas, and to determine what percentage of them are vol- 

 canic eruptive, what are massive eruptive. 



Upon entering into any description of the volcanic formations of any 

 locality, it becomes necessary to define the basis upon which it is done. 

 Discrimination in nomenclature of volcanic rocks has been sadly neg- 

 lected, and a number of terms have been applied without any acceptable 

 justification. Innumerable varieties, including texture and structure 

 as well as mineral constituents, increase the difficulty of arriving at any 

 definite conclusions, without a comprehensive knowledge of the entire 

 western volcanic products, while, on the other hand, the great variety of 

 specimens tends to mislead. 



Eichthofen, in his very able monograph On a Natural System of Vol- 

 canic Eocks,* refers the name trachyte to a definite order of rocks, 

 comprising within that order two lamiiies. The limitation of this term 

 that has previously been used indiscriminately for a number of trachoid 

 rocks was undoubtedly a step toward that improvement which is so much 



* Memoirs of the California Academy of ScieDce, 1^68, vol. i, part ii. 



