mil — Occitrrence of the Texas Mercury Minerals. 251 



Art. XXIII. — The Occurrence of the Texas Merciiry Min- 

 erals ; by Benj. F. Hill. 



The mercury deposits of Terlingua, Brewster County, 

 Texas,^ are found in both the Upper and Lower Cretaceous 

 rocks, wliicb in this locality are exposed in a section of more 

 than two thousand feet. In general the Lower Cretaceous is 

 made up of heavy thick-bedded limestones that are practically 

 free from foreign materiah The Upper Series, however, is 

 composed largely of thin-bedded marls, shales and impure lime- 

 stone with extensive clay beds. Both series are cut in many 

 places by old rocks, plugs and dikes of volcanic material, 

 the most common type of which are phonolite, andesite, and 

 basalt. The ore deposits are invariably within a short dis- 

 tance of these igneous manifestations. 



The deposits of the Lower Cretaceous, which in the present 

 stage of development are the most important, occur in a variety 

 of forms in the Edwards and Washita limestones. The most 

 common method of occurrence is in decomposed and brecciated 

 zones in the limestone. These zones are in many instances 

 contiguous to fissure veins, and it is probable that the ore-bear- 

 ing solutions were let into the broken and therefore receptive 

 zones through these channels. 



The fissures themselves often carry ore. They are invariably 

 calcite-filled. In no case in the whole Terlingua district have 

 quartz crystals in association with the ore been observed. 

 Besides the calcite are gypsum, iron oxides, manganese, and in 

 some localities much aragonite. 



The principal mercury-yielding mineral is cinnabar, which 

 occurs in a number of forms. Beautiful crystals of a ruby 

 red color, often three-quarters of an inch long, have been 

 found, intimately associated with calcite and native mercury. 

 The crystals are usually acicular prismatic, but at times have a 

 tabular habit, the prismatic condition being found only in 

 association with the calcite. Large quantities of cinnabar that 

 is crystalline occurs in granular masses, often of a large size. 

 These masses show distinct grains and under the microscope 

 exhibit crystal faces. The color of these granular aggregates 

 varies from bright vermilion to dark reddish-brown. The cin- 

 nabar occurs also in large amorphous masses which present the 

 same variation in color as do the granular masses. 



The native quicksilver is present in a number of openings 

 in the field, sometimes in considerable quantities. It is 



* Bulletin 4 of University of Texas Mineral Survey, Terlingua Quicksilver 

 Deposits of Brewster Co., by B. F. Hill. 



Am. Jour. Sci.— Fourth Series, Yol. XVI, No. 93.— September, 1903. 

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