8 Pi^'^CEEDINGS OF THE NATIONAL MUSEUM vol.74 



19.22, silica 5l'.\/J, alumina 1G.15, giving an approximate alumina- 

 silica ratio of 1 : 5. The mineral is therefore montmorillonite.^ 

 Small patches of diopside are scattered through much of this mate- 

 rial, or the diopside may be in such abundance that the resulting 

 rock is largely this mineral. In much of this pyroxenized material 

 the diopside is arranged in radiating groups. Small octahedrons 

 of magnetite are scattered through the groundmass and some streaks 

 carry abundant honey-yellow crystals of titanite. The groundmass 

 often carries later gypsum in small clear cleavage plates and some 

 dispersed calcite. Bluish-colored chalcedony is sometimes observed 

 and there are also occasional masses and veins of quartz visible. 



All gradations can be found from the slightly altered latite to 

 almost pure diopside rock so that there can be little doubt concerning 

 the nature and origin of this " pyroxenite." Plate 3 shows a breccia 

 of latite partially replaced by the dark-colored diopside. The resid- 

 ual latite fragments are now largely altered to montmorillonite. 



At the point called Labores de la Cueva a similar type of altera- 

 tion is encountered. The brecciated character of the rock is well 

 shown there. The dark-gray matrix of the fragments consists largely 

 of calcite and magnetite. (PI. 4.) Occasional crystals of diopside 

 are observed and rarely crystals of zircon. There are small veins of 

 calcite or vugs lined with the same mineral. The alteration of this 

 type is similar to that of the latite described above, but pyroxeniza- 

 tion is very much less pronounced and calcification much further 

 advanced. 



On the west slope of El Pedrigosa, immediately north of the main 

 ridge of Cerro Mercado, the rhyolite is extensively silicified, usually 

 with a complete obliteration of all structure of the original rock, 

 though rarely containing some of the original quartz phenocrysts of 

 the rhyolite. The material so silicified may be either compact and 

 porcelainlike or it may be soft and friable or again hard and vuggy. 

 Under the microscope the rock is fine grained and made up largely 

 of small radiating spherulites of chalcedony, with the interstices 

 filled with opal. Some quartz seems to be present, although the fine 

 grain of the aggregate precludes its definite determination. Many 

 of the individual grains are sharp cornered, suggesting that they 

 may be pseudomorphous after some other mineral, perhaps calcite 

 or adularia, but no unaltered mineral could be detected. This mate- 

 rial is mined and used for siliceous furnace linings. It consists of 

 97.7 per cent silica and over 2 per cent water, with other constituents 

 in very minor amounts. 



8 The Minerals of Beutouite and Related Clays and Their Physical Properties. Clarence 

 B. Ross and Earl V. Shannon. ~>urn. Amer. Ceramic Soc, vol. 9, No. 2, pp. 77-96, 1926. 



