76G H. S. WASHINGTON DECCAN SALTS AND PLATEAU BASALTS 



low because «of the fluidity of the lavas, and they always form a very 

 minor feature of the complex. 



Such lavas are called variously "fissure" or "plateau 1 ' flows. The term 

 "plateau" is used here because the word "fissure'-' connotes the mode of 

 origin, which is still somewhat uncertain. Although "plateau" is com- 

 monly connected with some degree of relative elevation above sealevel, 

 yet etymologically it implies only the ideas of flatness and horizontality, 

 which are characteristic of these flows. "Plateau" is used by Geikie, 

 Sollas (Suess), Salisbury, and others in speaking of these lavas. 



It is well known that the lavas of these plateau eruptions are mostly 

 basalt, and this petrographical character is the usually accepted explana- 

 tion of the great extension and horizontality of the sheets, since basalts 

 generally are known to be notably fusible at a lower temperature, and 

 more fluid when fused, than are more silicic lavas. But basalts vary 

 much, both chemically and modally, and many of them are evidently, on 

 extrusion, less fluid than are those of the plateaus. This is especially 

 true of the basalts of many volcanoes of the explosive type, the flows of 

 which do not extend very far and are often found consolidated on steep 

 slopes, as was pointed out nearly one hundred years ago by Lyell. 



It is purposed in this paper to take up the question of what character 

 may determine the peculiar fluidity of the plateau basalts as contrasted 

 with the more common ones. Especial attention will be paid to the 

 chemistry of the rocks, inasmuch as this appears to be the controlling 

 factor. It is not intended to enter into geological details or to undertake 

 studies of the areas dealt with. As I have not been able personally to 

 study the flows in the field, and am thus unable to give details as to the 

 relative positions of the various specimens in the series or other such 

 data, my study must be confined to a general one of the chemistry and 

 petrographical characters of the various specimens at my command. The 

 data thus brought together ma}', it is hoped, serve to supplement the de- 

 tailed field studies of future geologists in the various regions of plateau 

 basalts. 



The Deccan Traps 3 

 The Deccan traps 4 were extruded toward the end of the Cretaceous or 



3 This is the official designation of the formation adopted by the Geological Survey of 

 India (see Holland, Mem. Geol. Survey of India, vol. 43 (1), 1913, p. 40). The name 

 Deccan (from the Sanskrit dakhshin = souih) denoted originally '"that part of the In- 

 dian Peninsula which is south of the Vindhyan Range" (Holland). It is now commonly 

 restricted to the central elevated plateau of that region. 



4 For descriptions see Medlicott, Blanford, and Oldham : Manual of the Geology of 

 India, Calcutta, second edition, 1893, pp. 255-284 ; D. X. Wadia : Geology of India, Lon- 

 don. 1919, pp. 192-201; P. R. C. Reed: The Geology of the British Empire, London. 

 1 '.>•_' 1. p. 287. Figure 1 is k-ised on Oldham's map. 



