768 H. S. WASHINGTON DECCAN SALTS AND PLATEAU BASALTS 



The series of basaltic sheets is not of uniform thickness, reaching a 

 maximum of nearly 10,000 feet along the Bombay coast, being 2,000 to 

 2,500 feet thick at the southern end, 500 feet at the eastern extremity, 

 and only 100 to 200 feet in the Sind outlier. The average will probably 

 be between 2,000 and 2,500 feet. The average thickness of the individual 

 flows is about 15 feet, with maxima of over 50. Thin layers of ash and 

 some sedimentary deposits are intercalated between many of the flows, 

 especially in the upper part of the series. The flows are shown to have 

 been subaerial. The general horizontality of the sheets is a remarkable 

 feature, and only near Bombay do they dip slightly to the west. Very 

 rarely small flows of rhyolite and tracrn-te occur with the basalts, as in 

 Gujarat, at the northwest corner. These appear to be due to local differ- 

 entiation, 5 but they will not be discussed in this paper because no speci- 

 mens are at hand for study. 



Through the kindness of Drs. L. L. Fermor, Guy C. Pilgrim, and C. S. 

 Fox, the Geological Survey of India have sent me 23 specimens of the 

 Deccan traps, collected in past years from various localities widely scat- 

 tered over the basaltic area, and I take pleasure in expressing my thanks 

 to the Survey and to these friends for their generosity. Space will not 

 permit detailed descriptions of all the various specimens, so that more or 

 less general descriptions must suffice, leaving the full and detailed treat- 

 ment to the future work of the Indian petrographers. No detailed petro- 

 graphic descriptions of the Deccan basalts have been published. 



The Deccan basalts are fairly uniform in their megascopic features. 

 The specimens studied by me are all very dark gray or black, many with 

 a slightly brownish tint. They vary in granularity from very dense 

 aphanitic forms, with the semi-resinous luster of tachylitic basalt, 

 through very fine-grained forms (these being the most common), to 

 rather coarse-grained, doleritic forms. They are almost wholly aphyric, 

 the phenocrysts visible megascopically in a few specimens being rare, 

 small, equant crystals of glassy labradorite and a few small and rare 

 olivines in one or two. All my specimens are very compact, but Oldham 

 and Wadia state that vesicular forms are common. The splendid zeolites 

 of these amygdaloiclal lavas are well known in collections. Stilbite and 

 apophyllite are common, with less often scolecite, laumontite, and heu- 

 landite, all of these being calcic. Glauconite also occurs, as well as much 

 * I uartz and agate. 



Microscopically, these basalts show a rather remarkable uniformity in 

 mode or mineral composition. Plagioclase, chiefly a labradorite of about 



5 L. L. Fermor : Rec. Geol. Survey of India, 190G, p. 34. 



