260 CENTRAL MINERAL REGION OF TEXAS. 



mite, and yttrialite, described by Hidden and Mackintosh.* A much more 

 extended list would appear if all well marked varieties were included. The 

 foregoing species have all been collected and determined by the writer, be- 

 sides some undescribed species to be noted elsewhere in the report. Besides 

 these, Dr. Edgar Everhart, of the University of Texas, reports the occurrence 

 of columbite, samarskite, and phlogopite from Barringer Hill, but I have been 

 unable to detect these species in the material collected for the Survey. 



Pyrite, marcasite, and a number of iron, copper, and lead ores, as well as 

 gold and silver, occur among the strata of the Burnetan System, but they are 

 probably largely of more recent origin, being usually associated with infil- 

 trated masses, in streaks, veins, or pockets. The manganese ores seem also 

 to be of this secondary character. 



It is an extremely difficult matter to differentiate the rocks of this system, 

 and to draw sharp lines of demarkation, lithologic or structural, between the 

 great Archaean systems, which, in a general sense, seem clearly to be repres- 

 ented in this area. Any attempt at a classification, however acceptable the 

 result might be upon mere mineralogical grounds, must at present be largely 

 deficient in geognostic support; and again, if dynamic considerations be al- 

 lowed to prevail, the textural foundation is not assuredly supporting, except, 

 possibly, in a broad and rather untechnical view. 



The subsequent history of the region has been such as to commingle the 

 various trends in a most bewildering manner, and although each periodic 

 movement has seriously affected only a part of the area, as a rule — thus en- 

 abling one to discern fairly well the contemporary facts for each locus of dis- 

 turbance — the whole preceding record has usually been badly confused, if not 

 obliterated. Such a condition is often favorable to the special study of an 

 established series, but it is anything but conducive to safe generalization and 

 the correlation of disconnected outcrops. All that can now be properly an- 

 nounced upon this subject is comprised in the following broad statements: 



1. The rocks included in the most ancient uplifts are susceptible of a 

 rough separation into three compositional and geographic series, in which 

 there is no well ascertained warrant of overlap or unconformity. These are: 



(a) The basal gneisses and granites, with perhaps the felsite porphyry, 

 which may have formed the floor, now folded latitudinally, upon which the 

 later members of the series were deposited, f [Lone Grove Series.) 



*A Description of several Yttria and Thoria Minerals from Llano County, Texas. By W. 

 E. Hidden and J. B. Mackintosh. American Journal of Science (third series), vol. XXXVIII, 

 p. 474. 



f A very pretty theory, which is supported by some of my observations, but which can 

 not be said to be demonstrated as yet for this district, is that " the present schistose rocks 

 * * * were laid down upon the granitoid (Laurentian) gneisses, whatever may have 

 been the original lorm of these, before the main era of folding was inaugurated," This quo- 



