578 CENTRAL MINERAL REGION OF TEXAS. 



reasonable sums in development. In this way both sides assume some risk, 

 but both are equally protected against heavy losses of money or land. While 

 such a method is desirable in all mining operations in a new country, it is 

 particularly applicable to a region like this, in which there have been no pay- 

 ing mines; and more than all is it necessary for the mining of copper ores, 

 which must be obtained in fairly uniform grade and quantity in order to be 

 profitably worked. 



The hydrous copper carbonates {azurite and malachite) are the results of al- 

 teration of copper sulphides by the action of air and water, and these are 

 therefore not to be expected at very great depth in the workings. The sul- 

 phide ores are often less conspicuous to those unacquainted with their char- 

 acters, but they are liable to become the chief source of copper in this region 

 eventually. Although they may be regarded as more rebellious, there may 

 be an advantage in the fact that they can sometimes be concentrated by 

 washing before reduction, that mode of treatment being practically impossi- 

 ble with the carbonates. 



The copper fields of the Central Mineral Region are all within the areas 

 exposing rocks of the Burnetian System. Presumably anywhere that these 

 strata can be reached along two or more axes trending north 75° west across 

 the area there will be found evidences of the existence of copper ores. As 

 yet the explorations have commonly revealed only the oxidized and sulphur- 

 eted minerals, but it is not beyond possibility that some deeper source of 

 supply is a deposit of native copper. The chief reason for the suggestion of 

 such a possibility is, however, the analogy in point of age of the Burnetian 

 rocks to those which carry the copper in the Lake Superior Region. While 

 this may not be regarded as convincing evidence, it is strengthened by a very 

 significant structural feature, which was pointed out in my former report on 

 this region in the following words:* 



"The element of distribution is the north-south trend, and apparently a basic eruptive of 

 Post-Texian age is the exciting cause. * * * But it is not probable that the richest ores 

 lie in this trend at the surface; on the contrary the assays made for this Report give the 

 best record to the most ancient course, north 15° west, the one in which the silver ores 

 chiefly occur." 



The carbonates^ — azurite and malachite — occur in all parts of the copper 

 region, but the sulphides — bornite, chalcopyrite, etc. — are usually associated 

 with the silver ores. It is very significant that the copper districts are almost 

 invariably tracts in which both the Burnetian and the Texian uplifts have 

 marked expression. Exceptions to this rule are in line with pronounced axes 

 of both trends, although the two disturbances may not be readily traceable at 

 the surface. The Fernandian trend is also commonly manifest in the same 



*First Annual Report Geological Survey of Texas, 1889, pp. 334, 335. 



