280 



CENTRAL MINERAL REGION OF TEXAS. 



or a later set, may be included an apparently underlying series of thinly 

 laminated sandy shales, accompanied by peculiar sandy chloritic detrital sed- 

 iments. These are followed by hard, compact, white laminated quartz rock, 

 or quartzite, in beds associated somehow with ferruginous and schistose 

 layers, which may be of intrusive character. Bluish and green and ferru- 

 ginous shaly beds, in parts somewhat graphitic, but manifestly of detrital 

 origin, occupy a position which is probably higher than the quartzites; and 

 above these are limestones or marbles in beds of some possible economic 

 value. It is often difficult in the field to distinguish the graphitic shale and 

 marble, as a belt, from the similar lithologic set of the earlier Fernandan 

 System. In hand specimens, however, the distinction is obvious. The 

 Texan Beds are much less altered, as a rule. The graphitic strata are 

 plainly derivatives of the pre-existing graphite schists, and the marbles are 

 white or brown, instead of blue. 



In justice to Mr. C. D. Walcott, who first announced the existence of un- 

 conformity between Eparchaean and Cambrian strata in this region, his ap- 

 pellation of Llano is retained for that one of the Texan series which will 

 allow him this credit without seriously disturbing his later correlations. § 



TAXONOMY OF THE SYSTEM. 



Without intending any more permanent correlation than shall be justified 

 by further investigations, a provisional adjustment of the Texan taxonomy 

 seems to be reasonably indicated along the following lines: 



Group (Era). 



System (Period). 



Series (Epoch). 



Beds. 

 (Central Texas). 



EPARCHAEAN. ] 



TEXAN", ; 



or \ 



ALGONKIAN? 



3. Packsaddle (Chuar?) 

 2. Llano (Grand Canon?) 



1. Mason (Vishnu?) 



Marbles and shaly beds. 

 Quartzites and sand- 

 stones with eruptives? 

 Sandy shales and schists. 



IRRUPTIVES OF THE TEXAN SYSTEM. 



In the Mason Epoch there seems to have been in Central Texas a compar- 

 atively shallow 'sea extending westward from near the present meridianal 

 course of the Colorado River. The sediments of that littoral area are detrital 

 deposits such as could readily have been accumulated from the degradation 

 of the adjacent land of Laurentian age upon the east. The thin lamination 

 or stratification of these deposits, and the frequent interpolation of granular 



§ As a matter of credit, merely, the upper series might be the best thus designated ; but, as 

 Mr. "Walcott has in several published sections made the Llano Series parachronous with 

 Powell's G-rand Canyon Series, it will save future annoyance to use his term for our Middle 

 Series, as in this report. 



