1869.] CLARK INDIAN BASALT DYKES. 167 



earthwork ; and many traverse the country for miles together, 

 varying from one to two or three hundred feet in height, but 

 always narrow. Near Moorbar a number of these ridges inter- 

 sect each other, and form some very singular triangular enclosures. 



The dykes are observed not only near to the parallel of Bombay, 

 but they are seen in great numbers both north and south of that 

 line, and still present the same characters. A sufficient number of 

 dykes have been traced up from the Concan to the crests of the 

 Ghauts to make it clear that elevations of 3000 and 4000 feet pro- 

 duce no perceptible difference in their structure or density. 



The dykes are less numerous and of a smaller size as they recede 

 from the volcanic district. East of the Ghauts, therefore, they are not 

 numerous. Perhaps half a dozen have been noted between the Ghauts 

 and Poena, and none between Poena and Shoolapoor. N^orth of 

 Jooneer, or rather of the Hurreechunder range, they are said to 

 be more frequent, and to be found of large dimensions 50 or 60 

 miles from the sea. If this be so in these as well as in the more 

 eastern examples in the metamorphic districts, it seems probable 

 that such dykes are not to be traced to the Concan vents, but to 

 sources more to the eastward — sources independent above, though, 

 no doubt, identical below. 



It may be mentioned that where fissures in a lava stream have 

 been filled up by the overpouring of a later stream, the included 

 matter, probably from its contact with cooled surfaces, assumes a 

 denser structure than either its parent or including rock, it is, as 

 founders say, " chilled ;" and when the parent and superincumbent 

 mass is swept away, what is left much resembles a regular dyke. 

 Many such occur in the Deccan, but they are usually on a small 

 scale, zig-zag, and may be readily traced to an end. 



There seems every reason to believe that basalt dykes belong to a 

 very late, perhaps nearly the latest, period of general volcanic action 

 in the vast cape of India. The regular basalt beds which they re- 

 semble have not been observed lower than near to, if not at the very 

 summit of, the series upon the Ghauts, and at the actual summit in 

 Bombay — that is to say, on the two remaining margins of the volcanic 

 region. The Bombay basalt, indeed, seems precisely to resemble, 

 mineralogically, that composing the dykes, and to differ only in the 

 local accident of a more perfect rbomboidal and columnar structure, 

 due, no doubt, to its mass. The Ghaut sheet-basalt, weU. seen on the 

 great tableland upon and east of Beema Sunker, is rather pris- 

 matic than columnar, and it contains the honey-coloured mineral 

 and affects the needle. In the possession of these two qualities it 

 differs from the basalt of the adjacent dykes ; but I know not if this 

 be fatal to their general identity, or at any rate to their being re- 

 garded as of one general period. The nodular basalt common as a 

 capping-bed on the hiUs near Poena and the Beema is certainly cleft 

 by the dykes. Upon Beema Sunker, and, I believe, elsewhere at 

 great altitudes, beds of laterite, or of an amygdaloid much resembling 

 it, are found above the basalt-bed. I know not whether these beds 

 are reached or cleft by the dykes. 



