798 H. S. WASHINGTON DECCAN SALTS AND PLATEAU BASALTS 



of fluidity at the time of extrusion. The flows are individually of con- 

 siderable thickness and the total thickness of the series of superimposed 

 flows is very great. Ash beds and layers of scoria are not abundant. In 

 several regions the basalts are associated with flows of rhyolite or toscan- 

 ite, while accompanying andesite and trachyte are rarely met with, and 

 lenadic lavas, such as phonolite or tephrite, seldom or never occur. They 

 have been extruded at very different geological epochs, from the pre- 

 Cambrian to recent times. 



Megascopically, they are very dark, black or occasionally brownish 

 black, rarely dark gray. In granularity they may vary from rather 

 coarsely doleritic to densely aphanitic, some few being evidently highly 

 vitreous. Vesicular forms seem to be rare as compared with ordinary 

 basalts of volcanic cones. The great majority are aphyric, but there is 

 some tendency to a porphyritic development of the feldspar, especially in 

 the Thulean region, forming a special textural type. Augite seldom 

 forms megaphenocrysts, and these small and sparse, while olivine pheno- 

 crysts are very rarely present, except in some of the Algonkian and 

 Palisadan diabases. 



Thin sections show a striking Uniformity in mode, or at least in gen- 

 eral mineral composition. Augite and a labradorite (generally about 

 Ab 1 An 2 ) make up about 90 per cent of the rock in most cases, and in all 

 these two minerals form much the greater part; both are present in ap- 

 proximately equal amounts, although there may be some variation in the 

 preponderance of the one or the other. The augite is colorless or, more 

 generally, slightly brownish, and seems to be commonly an enstatite- 

 augite in all the regions — that is, the hypersthene molecule is present in 

 the pyroxene in amount about equal to that of the diopside molecule. 

 This is in strong contrast with common basaltic augites, such as the 

 loose crystals at Etna, Stromboli, Vesuvius, the Alban Hills, and Halea- 

 kala, which are dominantly diopsidic. The augite is almost always inter- 

 stitial, and consequently anhedral — a point to be mentioned later. 



The plagioclase offers no features of special interest, except that it is 

 practically always tabular and euhedral or subhedral. The ordinary 

 twinning lamellae are always present, zonal structure is rare or absent, 

 and it carries almost no inclusions. Orthoclase is seldom present ; so that 

 the molecule of this must exist in solid solution in the labradorite. In 

 the typical plateau basalts — indeed, in all the specimens studied by me — 

 no nephelite is present, although some nephelite basalts or tephrites ac- 

 company the normal basalts in a few places, as in the Thulean and 

 Siberian regions. 



Olivine is generally rare, except in the Algonkian and Palisadan re- 



