RARE MINERALS AND PRECIOUS STONES. 363 



2. BERYL. 



Some very large fine beryls have been found in Gillespie County, and oc- 

 casionally in Llano County. They occur in the Archaean in situations simi- 

 lar to those in which the garnet abounds. 



3. TOURMALINE. 



Black tourmaline is abundant in the Burnetan Granites (north 75 degrees 

 west trend), sometimes in large,, crystals, but it is rarely solid enough to 

 enable one to remove the mineral from the matrix without fracturing. It is 

 also so closely knit together with the coarse and interlocked feldspar crystals 

 that a blow upon the rock or the wedging of the feldspar causes the brittle 

 tourmaline to fall into small fragments. Often the jointage of the granite or 

 porphyry has cut through the tourmaline crystals as well. For any possible 

 uses of this mineral, where size and form are important, the quantity and 

 ease of working is not prohibitory, but there is no prospect that this field can 

 ever afford specimens of much value for cabinets. 



ESPECIALLY RARE MINERALS. 



A very careful search has been made by us for rare and valuable minerals, 

 and the results have been very satisfactory. So far as present developments 

 enable a judgment to be formed, the Burnetan outcrops seem to be the 

 sources of the most valuable varieties. There are several localities in which 

 the geologic conditions are identical with what have existed in the only tract 

 actually known as a commercial producer of the rarest minerals to-day. 

 Prospecting for such material has not been general, although there is every 

 reason to believe that many of the exposures will be found prolific if prop- 

 erly worked. 



A. THE BARRINGER TRACT. 



The discoveries thus far have been almost wholly confined to the limited 

 outcrop at Barringer Hill, in Llano County, upon the west bank of the Colo- 

 rado River, about four and a half miles south of Bluffton. This locality has 

 already been described in this Report, and a list of occurring minerals has 

 been given in the discussion of the rocks of the Burnetan System.* 



An engraving from a photograph is appended, the view being from the 

 river side, looking southwest war dly towards the excavation from which the 

 latest and best specimens of gadolinite have been mined. Lack of time and 

 press of other analytical work has prevented the proper study of these 

 minerals which were to have been described, and they must be left until the 

 next report. 



*See page — of this volume and the photographic view. 



