274 



CENTRAL MINERAL REGION OF TEXAS. 



But it is not possible now to do more than to block out roughly the lithologic 

 series represented, and similar gradations are possibly present in other parts 

 of the system. 



7. Overlying all is an important calcareous series showing textural altera- 

 tion, which implies much change since the deposition of the strata. The 

 same is apparent all through the Fernandan System; and while it may be 

 due in part to the intrusive rocks, that supposition is not sufficient to explain 

 all the facts of this kind, which are usually as prominent in one locality as in 

 another, irrespective of the distribution of the intrusives. 



TAXONOMY OF THE SYSTEM. 



It is not proper to offer any theory regarding the epochal mutations during 

 the Fernandan Period. Authorities differ greatly as to the arrangement and 

 terminology of the various Pre-Cambrian Series, and it is too early in our 

 study to make fast boundaries which better knowledge may oblige us to re- 

 tract. But a provisional classification seems essential to further progress in 

 this complex area.* It will also serve as a tool, perhaps only temporarily, 

 for elucidating the record. While it may be an error to place either the 

 Ontarian or our Post-Laurentian Fernandan System in the Archaean Group, 

 and while other imperfections will no doubt be discovered in my arrange- 

 ment eventually, I have found it impossible to adopt any other without risk- 

 ing the assumption of too much authority or too much favoritism. The 

 following schedule has at least the merit that it preserves the synchronic 

 order of deposition, as far as known, upon a framework which can be finally 

 adopted or rejected without affecting anything but the credit of an humble 

 investigator: 



Group (Era). 



System (Period). 



Series (Epoch). 



Beds.* 



ARCHAEAN. < 



Fernandan, 



or 

 Ontarian ? 



3. Click. 



2. Iron Mountain. < 



1. Valley Spring. -j 



7. Calcareous Rock. 



6. CMoritic Slates and Shales. 



5. Carbonaceous Schists. 



4. Eerruginous Rocks. 



3. Quartzites. 



2. Acidic Schists. 



1. Basic Schists. 



*The separation of the heds into series is based chiefly upon assumed similarities in the conditions of deposi- 

 tion, but the arrangement can not be considered very satisfactory. i 



* The writer will find it necessary to send assistants, with hammer and compass, to trace 

 out and map in detail many points of structure here only indicated, and this work can 

 hardly be accomplished without some such preliminary skeleton as is here given. It is be- 

 lieved, however, that all who are interested in this part of the report will have no difficulty 

 in distinguishing between the facts themselves and the interpretations put upon them here, 

 tentatively. 



