

HISTORY OF LEUCITE BASALT. 283 



Shoshone Mesa the basalt contains quartz, and occasionally augite is 

 deficient or almost wanting. The olivine crystals are often so abun- 

 dant as to give a cleavage to the rock. Occasionally they form nearly 

 the whole ground mass, and the crystals often contain an abundance 

 of picotite. 



On the south of Black Eock Mountain, in Mud Lake Desert, is a 

 coarse dolerite in which the plagioclase crystals are an inch long, and 

 the augite crystals a fourth of an inch in diameter. The ground 

 mass of this rock is slightly vitreous. 



Aspect of Basaltic Rocks. In colour basalt may be black, choco- 

 late, dark grey, or greenish, but the latter colour is always due to abun- 

 dance of olivine. It frequently exhibits horizontal columnar structure 

 indicative of having cooled in dykes. It sometimes occupies eroded 

 valleys in the older rhyolite. Where decomposed, the basalt is charged 

 with seladonite and chalcedony. It shows all stages of texture, from 

 compact crystalline, through porous, to highly vesicular and scoriaceous 

 rocks as light as a sponge. Basaltic tuffs are met with, and sometimes 

 form an earthy and sometimes a true pelagonite. Even those which 

 are compact vary in texture ; some basalts are evenly granular and 

 poor in glass, like the common British types. Others have a ground 

 mass formed of a very fine crystalline aggregate of microliths of felspar 

 and augite, with larger crystals of felspar and olivine, and occasionally 

 of augite also. A third type is a homogeneous yellowish-brown glass 

 half filled with crystals, while sometimes the basalt is a mixture of 

 small and large crystals, with wedge-shaped masses of globulitic glass 

 between the crystals, and is then regarded as closely related to augite 

 andesite. Many of the American basalts closely approximate to those 

 of Europe. The basalt near American Elat Creek in Washoe, like 

 that of Scheinnitz in Hungary, has no glassy base, and consists of an 

 aggregate of fine-grained, pale-coloured augite and black magnetite, 

 with macroscopical and large microscopical crystals of plagioclase and 

 sanidine, often with olivine. But the minerals undergo considerable 

 change with the locality. East of Spanish Spring Station, in the 

 Virginia range, the augite is green. Near Wadsworth the plagioclase 

 includes augite and olivine. The olivine itself often contains octa- 

 gonal crystals of picotite, a variety of chrome spinel, also found in 

 the basalts of Germany, Bohemia, Hungary, and Italy. The nephe- 

 line basalts contain tridymite and sanidine. On the Upper Little 

 Snake river grains of quartz occur surrounded by augite. 



Leucite Basalt. 



Leucite basalts consist of augite and leucite. Mica occurs in 

 microscopic films in the fine-grained ground mass which contains 

 porphyritic crystals of augite, and usually olivine. The rock is very 

 rarely glassy, though a glassy condition is seen in the magma of the 

 Vesuvian lavas of 1822 and 1858. The basalt of Schackau in the 

 Khon Mountains is particularly rich in leucite. On both slopes of 

 the Erzgebirge the basalt consists chiefly of leucite, augite, and 



