BASALT FAMILY 201 



of alkalies is small, while that of iron, magnesia, and lime is much 

 greater. They are heavy, dark-coloured rocks, which generally 

 weather red, from the oxidation of the FeO which they contain. 

 The principal minerals are a plagioclase felspar, rich in lime 

 (labradorite or anorthite), some kind of pyroxene, magnetite, 

 and frequently olivine. There is a wide range of mineralogical 

 composition and many varieties of rock occur in this family, but 

 often these can be distinguished from one another only by the aid 

 of the microscope. 



Tachylyte is a basaltic glass, which is not at all common. 



Basalt is a name of wide application covering many varieties, 

 which, however, can seldom be distinguished by the unassisted 

 eye. They are very common volcanic rocks, and most of the 

 active volcanoes of the present day extrude basaltic lavas. In 

 texture the basalts are ordinarily porphyritic, but they may be 

 without phenocrysts, and consist of a finely crystalline mass. The 

 ground mass is made up of tiny crystals, mingled with a dark 

 glass. 



The basalts are closely related to the andesites and connected 

 with them by a number of transitional forms, but in the andesites 

 the phenocrysts are principally felspars, which is not the case in 

 the basalts. Those basalts which contain olivine in notable quan- 

 tities are called olivine basalt; while those in which the felspar is 

 replaced by leucite or nepheline are called leucite and nepheline 

 basalt, respectively. Several other named varieties occur, but, 

 for the most part, they require the aid of the microscope for 

 their identification. A rare variety, found in New Mexico, Cali- 

 fornia, and elsewhere, contains phenocrysts of quartz. 



Trap is a useful field name for various sorts of dark, granular 

 rocks, which cannot readily be distinguished by inspection. The 

 term is often applied to diorite and especially to diabase. 



Dolerite is a coarsely crystalline basaltic rock, which is either 

 porphyritic or granitoid in texture. 



Diabase is a rock of peculiar texture ; the felspar crystals are 

 long, narrow, and lath-shaped, and contain the dark minerals in 

 their interstices. The trap rocks of the Palisades of the Hudson, 



