Precious Gem stones production in 1913 Page 91



	684 MINERAL RESOURCES, 1913----PART II.

	for cutting into gems but the most of it is worth more as specimens, because of the perfection of the crystals. They range from colorless to pale bluish green, and some are yellow from iron stains on the surface. Phenacite was observed in small transparent colorless crystals only, which were attached to or partly imbedded in feldspar, quartz, or topaz. Crystals of phenacite measuring over hah an men across are reported to have been found. The phenacite crystals from Baldface Mountain have been described by O. C. Farrington¹ and W. T. Schaller.² At the lower prospect on the northeast shoulder of Baldface Mountain, topaz occurs in a thin pegmatite vein cutting the granite with a strike of N. 35° W. and a dip of 45° SW. This vein has been exposed for about 75 feet by the dropping away of the northeast side attached to a huge block of granite. Crystals have also been obtained from the vein attached to the loose block about 50 feet lower down the mountain side. The vein contains fine graphic granite in places and small miarolitic cavities with mineral associations similar to those at the first prospect described. TEXAS. The occurrence of topaz in Mason County, Tex., has been men-' tioned in these reports for previous years and some of the deposits have been described by H. Conrad Meyer.³ Several of the localities were visited in July, 1913, when the notes for the following description were obtained. The discovery of topaz was made in 1904 by R. L. Parker. Other prospects were found and all were developed on a small scale between 1908 and 1910. The owners of the different prospects failed to realize on the topaz found on their property and therefore forbade further work. Since that time a limited 'amount of prospecting has been done by the property owners, in some places with promising results. Meyer first recorded the occurrence of cassiterite or stream tin associated with the topaz of Mason County. Mason County is in the broken plateau country of central Texas. Much of the county is a basin-like area with included ridges and hills. The county is drained chiefly by Llano River, on which the lowest elevation is less than 1,200 feet above sea level. The topaz localities examined are at elevations of from 1,600 to 1,800 feet above sea level. Water is scarce and only the larger streams flow the whole year. The basin-like portion of Mason County is part of the pre-Cambrian basin described t>y Sidney Paige.⁴ The surrounding rim rock or scarp is part of a dissected plateau composed chiefly of Paleozoic rocks, among which limestone is prominent. The pre-Cambrian rocks in the topaz areas consist largely of granite, but a belt of rock composed of mica schist and mica gneiss was observed between the two groups of deposits. The principal granite of the topaz region is a coarse red granite, with porpnritic texture locally developed. The feldspar phenocrysts of this granite measure over an inch long in some places. A finer grained granite, also reddish, was observed in the region but not associated with the topaz deposits. Pegmatite is ¹ Notes on various minerals in the Museum collection: Field Columbian Mus. Pub., Geol. Ser., vol. 8, pp. 157-158,1908. ² Notes on crystallography of phenacite: U. S. Geol. Survey Bull. 490, p. 53,1911. ³ Topaz and stream tin in Mason County, Tex.: Eng. and Min. Jour., Mar. 8,1913, pp. 511-512. * TJ. 8. Geol. Survey Geol. Atlas, Llano-Burnet folio (No. 183), 1912.