598 CENTRAL MINERAL REGION OF TEXAS. 



There is an interesting parallelism or symmetry in the arrangement of the 

 mineral belts within the Burnetian tract, which goes far to prove the original 

 continuity of the broken axis across the whole tract. So far as my observa- 

 tions go* the cassiterite and the minerals which carry tin in minute quantities 

 are confined to the northern half of the tract, the cassiterite itself being ap- 

 parently within the southern half of this portion. Garnet seems to occur in 

 two belts lying respectively north and south of the distinctive tin field, but 

 there is also a medium garnet tract which corresponds more or less closely 

 with the central belt, or the tin and gadolinite area. Tourmaline is chiefly 

 characteristic of the southern half of the whole tract, and keilhauite appears 

 to cling to its vicinity in a measure. 



Epidote is an abundant northern representative; labradorite is scarce and 

 confined to the southern half; voigtite, hitherto unreported, is of the medium 

 belt, in which cassiterite, gadolinite, fergusonite, cyrtolite, allanite, and other 

 rare minerals occur, and idocrase is also thus distributed. In this central 

 band albite, fibrolite, opaque white quartz, and what may be called "graphic 

 orthoclase" are striking features. Quartz mounds, some known to contain 

 similar minerals to those at Barringer Hill, are also characteristic of the 

 middle portion. 



A feature of the northern third of the region is the occurrence of the ores 

 of copper and of silver (sparingly), which are much less characteristic of the 

 southern tract and of rather local distribution in the median belt. 



Much of the area in which cassiterite may reasonably be sought is now 

 deeply covered by detritus and no prospecting or exploration has been done 

 as yet. The Burnetian rocks are likewise exposed southward in portions of 

 Blanco and Gillespie counties, where equivalent features are apparent, but 

 the complexity there is such that generalizations as to distribution are much 

 less satisfactory. 



The percentage of tin in such minerals as contain but traces in the north- 

 ern tract is sufficient to give the red borax bead with copper oxide in the 

 lower reducing flame, and with greater saturation a copious reduction of the 

 sub-oxide. In the solution nitric acid sometimes gives a rather heavy white 

 precipitate, but with soda and cyanide and considerable borax no appreciable 

 reduction of metallic tin is usually possible, excepting in the cassiterite. 



More than thirty years ago Mr. J. Geo. Durst discovered the remains of 

 two old furnaces about three miles apart, each of considerable size. From 

 one of these, near the head of Willow Creek, in Mason County, a large 

 amount of matte and slag has been taken (so Mr. Durst says) in the belief 

 that silver bearing ores had been smelted there by the Spaniards in early 



*I have traversed and mapped the whole region in such manner that no important out- 

 crop can have been overlooked. 



