252 CENTRAL MINERAL REGION OF TEXAS. 



SUMMARY. 



Thus it will be seen that the present writer entered the field with a number 

 of unsolved problems in hand, with a large area practically unexplored, and 

 with not a little conflicting testimony in print regarding questions which had 

 been for the most part locally and superficially examined; partly by inexpe- 

 rienced and untrustworthy investigators, but also by several well known and 

 competent authorities, whose conclusions, if incorrect, must have been due to 

 imperfect knowledge of the facts. 



If we attempt to select the most reliable evidence, it will be found discon- 

 nected in area and so little supported by detailed stratigraphic work that only 

 a small portion of what has been heretofore published can be depended upon 

 as the basis of any but the most crude generalizations. The work of Rcemer, 

 as a very general outline of the geology; the careful but unappreciated and 

 poorly presented discoveries of Shumard; the unpublished results of Glenn, 

 unfortunately not now accessible; the hasty examination of Walcott, as far 

 as it goes; and the study of Hill (as it affects this region), are all that can be 

 utilized for any scientific purposes; and very much of this material can not 

 be made to tally with the discoveries made by myself in the detailed study of 

 the whole region in the year 1889. 



The best conclusions which can be drawn from a study of the literature 

 alone up to date are as follows: 



1. That there is a large area of granite and other crystalline rocks in the 

 district, more or less confused by disturbances of uncertain age or ages. 



(a) That Walcott believed there are no Archaean rocks among these crys- 

 tallines, but 



(b) That his observations in 1884 were too limited in area to make this 

 conclusion final. 



2. That a group of rocks beneath the Potsdam, and including at least a 

 portion if not all of the crystallines which were not eruptive, have been re- 

 ferred to the Paleozoic group by Walcott, under the name of Llano Series. 



(a) That he considers this pre-Cambrian and equivalent to a portion of the 

 Algonkian system. 



3. That Rcemer, Shumard, Walcott, and all persons whose opinions are 

 entitled to credence, have reported Cambrian and Lower Silurian rocks over 

 a comparatively wide area in the district. 



(a) The Potsdam (Upper Cambrian) is reported by all who have worked 

 in the field. 



(b) The Calciferous division, or its equivalent, is generally recognized in 

 the writings of competent observers. 



