322 CENTRAL MINERAL REGION OF TEXAS. 



broken exterior, they were more or less rounded, and exhibited a gradual slope to the 

 prairie level, while the granitic structure almost entirely disappeared, its place being occu- 

 pied by fine porphyry of a reddish color. 



July 18. — * * * We arrived at Cache Creek. * * * From the water's edge rose 

 abruptly a long line of smooth perpendicular cliffs, varying in height from 300 to 400 feet, 

 and having in some places a slight columnar structure. * * * Upon examination they 

 were found to be composed mostly of fine porphyry of a reddish color, which was traversed 

 by parallel and nearly perpendicular veins of cellular quartz, varying in thickness from two 

 to three feet. 



July 19. — About one mile below our present encampment 1 came to the termination of 

 the cliffs. A short distance below this I observed a nearly horizontal stratum of coarsely 

 laminated sandstone, fifty feet thick, and including in its composition fragments of igneous 

 rock of the same character as that composing the cliffs, the intermediate space being occupied 

 by red clay, which as before appeared to underlie the sandstone. 



These statements, and the view and section given in Marcy's report, de- 

 scribe fairly the situation in the southeastern portion of the Wichita Range. 

 The sandstone referred to as overlying the Permian red clays is what Mr. 

 Cummins calls the Fort Sill Series, and our observations make me think it 

 may be Tertiary. It is persistent along the southern base of the Wichitas 

 for many miles westward, and in most places it is overlaid by thick deposits 

 of Quaternary gravels and boulders of local origin. 



The porphyry outcrop of Carlton Mountains strikes in a general way east and 

 west, and a close examination of its structure shows that its eruptive line was 

 probably in the path of the great Post-Carboniferous uplift. This trend is dis- 

 cernible in almost every part of the Wichita Mountain System in the form 

 of joints, and, in every case thus far observed, it breaks all other joint-courses 

 except that trending northeast. This is in exact accordance with the facts 

 in Central Texas. 



The texture and the columnar structure of the igneous mass, its peculiar 

 topography, and the mode of deposition, as well as the subsequent erosion, 

 all point to a volcanic origin. The greatest development was off to the west, 

 near where the ridge seems to merge into the main chain southwest of Mount 

 Scott, and here^ as in a portion of the eastern end, there is all the structure 

 and resulting topography of an ancient crater. The slopes and semi-stratifi- 

 cation give the appearance of lava flows, and this feature is a very common 

 characteristic of the Wichita rocks of different ages. 



North of these lava hills, which include the Palisades of Cache Creek 

 above Fort Sill, there is a ridge, or a long series of irregularly disposed peaks, 

 which forms the backbone or the real geologic axis of the Wichita chain. 

 Mount Scott and its eastern neighbor, for which I here propose the name Mount 

 Cummins, as well as other peaks still further east and a multitude of knobs 

 and peaks extending many miles westward, all belong to this nucleal trend. 



