CHAPTER VIII. 



ERUPTIVE BOCKS — TRACHYTES — TRACHORHEITES — BASALT. 



I separate the volcanic areas of the district into three divisions, 

 according to the character of the rocks covering them : first, the porpby- 

 ritic trachyte forming the group of mountains marking the western or 

 southwestern termination of the Elk Mountains in the southeastern 

 portion of the district; second, the trachytic areas (mostly rhy otitic 

 underlaid by breccia) which form a large part of the southern portion ; 

 third, the basaltic areas that prevail in the northern part. 



The rocks of the second division correspond to the "Trachorbeites" 

 of Dr. Endlich, and in fact are the northern and western extension of 

 those rocks described by him in the report for 1873 under section c* 



With the exception of the first division, the volcanic rocks form the 

 tops of plateaus which have been much modified by erosion. These 

 plateaus are generally covered with a luxuriant growth of grass and 

 scattered groves of cotton woods. 



The volcanic rocks of the first division form beautiful isolated mount- 

 ain masses, surrounded with sandstones of Cretaceous age. 



The rocks of the third division are probably of most recent age, 

 although it is difficult to say definitely that they are more modern than 

 the rhyolitic rocks in the southern part of the district. The latter pre- 

 vail largely in the district assigned to Dr. Endlich, and he probably 

 secured more evidence as to their age than I was able to. 



PORPHYRITIC TRACHYTES. 



Although the rocks that I shall describe under this head differ con- 

 siderably from each other, they have the same general constitution. 

 They contain the same minerals, and are generally of light-gray colors, 

 with crystals of feldspar porphyritically imbedded in the mass with 

 hornblende and occasionally mica. They resemble the rocks found 

 throughout the Elk Mountains in 1873, especially those found at Gothic 

 Mountain and the various dikes found penetrating the sedimentary 

 formations. Some of the more compact varieties have a granitic appear- 

 ance, reminding one of the rocks forming the central masses in the main 

 mass of the Elk Mountains. 



The physical features of the country in which these mountains are have 

 been fully described in previous portions of the report, and I therefore 

 simply refer to them here. 



The isolated character of the peaks is well shown in Plate XI. a, &, 

 and c represent some of the trachytic peaks, which stand like huge 

 monuments in the midst of the Cretaceous sandstones which are seen 

 outcropping in the bluff in the foreground of the picture. At d and e 

 is shown a dike of trachyte, which once formed one continuous mass. 



* Report U. S. Geol. Survey 1873, page 343. 



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