DESCRIPTIVE GEOLOGY. 677 



CHAPTER II. 



DESCRIPTIVE GEOLOGY. 



Until the present time the closer examinations in the Trans Pecos country 

 of Texas have been confined to the extensive mineral districts of El Paso 

 County. Before going into geological details it is absolutely necessary to 

 have reliable topographic maps, which, with the limited assistance at my dis- 

 posal and the extraordinary difficulties which have to be overcome, is slow 

 work. 



There is no summary way to deal with the intricate geological problems of 

 this part of the State. The only short and easy method of getting out of the 

 complications and difficulties from the fragmentary observations made up to 

 the present time might be to take suppositions for facts; to base conclusions 

 on suppositions and fictions, and to tell what we do not know and can not 

 know about it. Therefore I shall only attempt to give a general description 

 of the geological features of a part of Trans-Pecos Texas, basing this descrip- 

 tion on the closer observations mentioned above, on the observations of a 

 flighty reconnoitering trip, made in behalf of the State Geological Survey 

 during the winter of 1888 and 1889, and on the experience of a number of 

 private exploring excursions which I had made before that time (since 1878) 

 to different localities in the western mountains. 



The most northwestern mountain range of Trans-Pecos Texas is located 

 north of El Paso and known as the " Franklin Range." It extends from El 

 Paso in a northern direction about twenty miles, and although isolated by a 

 depression from the Sierra Soledad of the Organ Mountains in New Mexico, 

 it must be regarded as a continuation of this range. 



The southern part of the Franklin Mountains is limestone overlaid by a 

 quartzitic rock (metamorphic, semi-fused sandstone) which rests on granitic 

 and porphyritic rock, with intrusive basaltic dykes. This southern part dips 

 towards El Paso (south). Farther north we find sandstone and shale strata 

 of considerable thickness dipping north, and it seems that the granitic and 

 porphyritic peaks about midway between El Paso and the gap between the 

 Organ Mountains and the Franklin range must be regarded as the eruptive 

 centre of the Franklin Mountains. 



The limestone capping of the granitic and porphyritic rocks is, according 

 to Prof. Shumard, of Silurian age. which seems confirmed by fossil frag- 

 ments found there, among which is distinctly recognizable Halysites catenula- 

 tus. East of the Franklin range is a flat ("The Mesa") about twenty-five 

 miles wide, extending towards the Hueco Mountains and northward into New 

 51— -geol. 



