588 CENTRAL MINERAL REGION OF TEXAS. 



The rocks have been plicated, crumpled, twisted, and metamorphosed until 

 all semblance of order has been obliterated, excepting that the successive 

 trends characteristic of the various uplifts are all discernable in their normal 

 chronologic sequence. But the system worked out and portrayed in the plan 

 and section is by no means as apparent to the eye of a casual observer. In 

 such a field systematic mining is not among the possibilities. The confusion 

 is of course due to the crossing of the several axes at this point, and it may 

 be that somewhere in the neighborhood, in a less jumbled area, the lead ore 

 may be more accessible, in comparatively better masses. As yet the results of 

 explorations have not been wholly encouraging. The faults shown in Figure 

 67 are very apparent also in the surrounding country, into which they radiate 

 from this centre, and it is probable that the latest of these have had a very 

 important influence in dislocating any possible segregations of commercial 

 value. For the present, therefore, it will be most advantageous to prospect 

 only in the least disturbed localities, or at least in those in which the old Si- 

 lurian axis (north 25° east) has not been much obscured by the later lines of 

 uplift. 



REVIEW OF THE LEAD BEARING ROCKS. 



After close attention to the whole subject of the nature and mode of oc- 

 currence of the lead ores in this region, the conclusions here announced are 

 the best that can be given at this writing. 



Character of the Ore. — The lead ore is almost wholly galena, in cubical 

 crystals of comparatively small size. In some cases a meagre incrustation of 

 lead carbonate of oxide is apparent in the limestones, but no important de- 

 posits of this nature have been discovered. There are usually no other min- 

 erals associated with the galena, which is scattered through the containing 

 rock without regularity, and commonly not in any considerable abundance. 



Mode of Occurrence, etc — Until now there has been no clear under- 

 standing of the origin of the lead deposits. In my Report last year* the 

 information then gathered was summarized as follows: 



We know that the galena occurs in beds that lie somewhere between the base of the 

 Potsdam limestone and the Deep Creek division of the San Saba series, and it is more than 

 probable that the horizon is very close to that of the Galena limestone of other States. 



One of the difficulties connected with the determination of the geologic 

 horizon of the lead bearing limestones is the fact that the sedimentary beds 

 are mainly shallow water deposits, and consequently their irregularity is a 

 prominent characteristic. Again, as in every case yet observed there is a 

 close connection of the lead accumulations with disturbances of the strata and 

 with irruptions of granite, it is not easy to make out whether the original 



*Page 341. 



