PETROLOGY 327 



Only two representatives of the sodipotassic subrang ourose of the rang' 

 limburgase are listed in Washington's 14 tables. Both of these are de- 

 scribed as monchiquite; one (column IV), by Weed and Pirsson, from 

 Willow Creek, Castle Mountains, Montana; the other (column III), by 

 Hunter and Eosenbusch, from Eio do Ouro, Serra de Tingua, Brazil. 



OLIVINE DIABASE (AUVERGNOSE) 



Megascopic character. — Of the five types of igneous rocks thus far iden- 

 tified in this region, diabase including basalt, is much the most frequent 

 and abundant in occurrence. Careful study of the rock in its numerous 

 field occurrences and in thin-sections under the microscope reveals re- 

 markable uniformity without trace of unusual feature in any one of the 

 many exposures examined. In every respect — general physical appear- 

 ance, texture, and mineral composition — the rock is identical with the 

 normal Triassic diabase of the Piedmont province. 



Naturally, some variation in the size of grain is observed in different 

 dikes and in different parts of the same dike in the larger ones, the range 

 in granularity being from medium to fine grained (aphanitic). Rarely 

 are any of the minerals in the coarser grained specimens sufficiently large 

 to be termed phenocrysts, yet in hand specimens it can usually be seen 

 that the rock is composed of both dark and light colored minerals. The 

 fresh rock is dark bluish gray in color, but those specimens in which 

 olivine is abundant the rock usually has a greenish cast. 



Microscopic character. — Thin-sections of all the field occurrences of 

 diabase noted by the writers were studied microscopically and were found 

 to be remarkably uniform in texture and composition, without variations 

 from the normal type of the Triassic olivine diabase. Without exception 

 the texture is typically diabasic or ophitic, the lath-shaped crystals of 

 plagioclase measuring in some cases as much as 3 millimeters in length, 

 but the average is probably less than half of this. From a microscopic 

 study of specimens of basalt collected by Darton from the dikes in High- 

 land County, Virginia, Keith 15 states that there are three distinct types 

 of texture — basaltic, diabasic, and porphyritic. In 1889 Diller 18 exam- 

 ined microscopically four specimens of similar rocks collected by Darton 

 from the same county and remarked that "all are of the holocrystalline 

 porphyritic type of basalt." 



In mineral composition the rock is an olivine diabase, although the 

 amount of olivine present is subject to considerable variation. In most 



14 Professional Paper No. 14, U. S. Geological Survey, mo;;, pp. 342-348. 

 ''Ainer. Jour. Sci., 1808, vol. vi, p. 314. 

 10 Ibid., !*<)<>, vol. xxxix, p. 270. 



