Geology and Mineralogy. 251 



12. Occurrence of Tin in Central Texas ; by Theodore B. 

 Comstock. (Communicated.) — The recent newspaper reports of 

 the existence of tin ores in Central Texas have had origin in the 

 official announcement in November by the State Geologist of the 

 discovery by the writer of good specimens of cassiterite in several 

 localities in Llano and Mason Counties. As yet no workable 

 deposits have been found, but the indications are such as to 

 encourage prospecting, if conducted in accordance with the 

 knowledge of the structure acquired by the Geological Survey. 



The cassiterite has all been found in a belt of the lowest 

 Archean (Burnetan) rocks, trending N". 75° W. through Llano 

 and Mason Counties, with a short extension eastward across the 

 Colorado River into Burnet County. In many places this field is 

 complicated by the effects of later upheavals, the structure in 

 detail having been described in the First Annual Report of the 

 Texas Geological Survey.* Some very interesting observations 

 concerning the associations of the tin ore, its special area of dis- 

 tribution along a single axis of the Burnetan system, and its 

 relations to an interesting series of rare minerals can only be 

 hinted at here, but they will be shortly announced in the 2d Annual 

 Report now in the printer's hands. The writer has made a thor- 

 ough examination of the district and has prepared accurate maps 

 and sections to illustrate the stratigraphy. Yery much of the 

 material which has been reported as tin ore from this district is 

 keilhauite, tourmaline or black garnet, but the cassiterite has been 

 collected by members of the writer's Division of the Survey from 

 four localities besides the one in which an old furnace was dis- 

 covered with tin globules in the slag dump. 



Austin, Texas, Jan. 24, 1891. 



13. Brief notices of some recently described Minerals. — Pina- 

 kiolite is a new borate of manganese and magnesium described 

 by Flink from Langban, Wermland, Sweden. It occurs in small 

 orthorhombic crystals imbedded in limestone. Cleavage brachy- 

 pinacoidal ; hardness = 6 ; sp. gravity^ 3*881 ; color black. An 

 analysis, alter deducting impurities, gave: 



B 2 3 Mn 3 4 Fe 3 4 MgO CaO PbO 



16-05 50-63 2-12 29-30 1-12 0'78 = 100. 



For this the formula 3MgO . B 2 3 + MnO . M 2 3 is calculated 

 which brings it into relation with ludwigite and sussexite. — Zs. 

 Kryst., xviii, 361, 1890. 



Trimerite is a silicate of beryllium, manganese and calcium ; 

 also described by Flink (1. c.) from the Harstig.mine, Wermland. 

 The crystals have a hexagonal form, but are shown optically to 

 be pseudo-hexagonal and twins of triclinic individuals. Cleavage 

 basal; hardness 6-7; sp. gravity=3'474 ; color pink. An anal- 

 ysis gave : 



Si0 2 BeO MnO FeO CaO MgO 



39-77 17-08 26-86 3'87 12-44 0-61 = 10063. 



* Preliminary Report on the Geology of the Central Mineral Region of Texas. 

 By Theo. B. Comstock, F.G-.S.A. In 1st Ann. Rep't of State Geologist, 1889, pp. 

 255-267, etc. 



