562 CENTRAL MINERAL REGION OF TEXAS. 



3. TEXIAN (ALGONKIAN) SYSTEM. 



Walcott's Algonkian System, composed of Powell's Vishnu, Grand Canyon, 

 and Chuar Series, is in many respects parachronous with our Texian System, 

 which includes three very similar series of strata. In Central Texas the pri- 

 mary trend of the series is within less than 1° of a true north-south course, 

 or about north 10° west by the uncorrected compass bearing. As with the 

 earlier systems, this has a particular region for its best expression, and in dis- 

 tricts not too much disturbed by more recent upheavals it is easy to distin- 

 guish the characteristic strata. They are sometimes tilted at high angles, 

 but frequently less than 65°, down to 30° or less. Many of the rocks have a 

 general resemblance to some of the Fernandian members, but it is usually 

 possible to detect signs of wear in the particles, implying an origin in part 

 from the degradation of the earlier formed terranes. Shales, sandstones, or 

 quartzites, and clear white crystalline marbles are characteristic The areas 

 of best exposure are in the eastern half of Mason County and in the south- 

 eastern corner of Llano County. The most important economic feature is 

 the occurrence of a very interesting class of minerals carrying rare metals. 



MASON SERIES. 



The sandy shales and schistose rocks which characterize the Mason series 

 are such as might readily be formed from the erosion of the gneisses and 

 schists of the Burnetian System, which were well exposed where these were 

 deposited. They are especially prominent in the upper valley of Comanche 

 Creek and in much of the region about Mason, more especially northward 

 from that town. 



LLANO SERIES. 



This was the series first reported by Mr. Walcott as seen by him in sub- 

 contact with the Cambrian sandstone of Packsaddle Mountain. But there it 

 is not in the axis of a Texian uplift, owing to important subsequent upheavals 

 which have obscured the earlier history. Good exposures in the character- 

 istic north-south trend occur farther southwest at the eastern base of the Riley 

 Mountains and at the northern edge of the same irregular range, a few miles 

 west of Sharp Mountain. The rocks are granular quartzites, or sandstones, 

 and platy quartzite rocks of more dense texture, with green schists alternat- 

 ing with them in some sections. The intersecting belts of glistening iron ores 

 near Fly Gap, Mason County, at the source of Herman and Willow creeks, 

 are probably in this terrane, although their occurrence is due to special and 

 local influences. 



