300 Dr. R. D. Thomson on the [April 



times under it, both being traversed frequently with basaltic 

 dykes. All these rocks at the shore may be observed 

 covered with shells, and their surface with a decomposed 

 powdery matter, which on inspection turns out to be the 

 basis of the rock crumbling into dust. At the distance of 

 two miles from the town, many of the shells which abound 

 so profusely on the sea-shore are calcined, by which pro- 

 cess, they are converted into caustic lime, which the natives 

 term chunam, a substance in great request, both as a mor- 

 tar, and as an edible rolled up in the betel leaf. The latter 

 habit, which has been denominated by some writers a 

 luxury, ought rather to be termed a necessary practice, as 

 we find it prevailing wherever the sole articles of diet are 

 procured from the vegetable kingdom, the different sub- 

 stances employed fulfilling the same end, whether it be on 

 the coral rocks of the Pacific, the arid deserts of Africa, or 

 the interminable forests of America. 



SALSETTE ISLAND. * 



The description of the geological structure of one of these 

 islands, may be said to include almost the particulars of 

 the whole, but for the sake of greater perspicuity, we have 

 ventured to consider them separately. The essential com- 

 position of Salsette, is clay stone porphyry and amygda- 

 loid, corresponding with those rocks in Bombay, but basalt 

 occurs in very considerable tracts, and assumes more de- 

 cided forms. The island is very irregular in its surface, 

 consisting of ridges and intervening vallies, which in com- 

 bination afford agreeable scenery. 



The basalt forms two ridges which run parallel to each 

 other, the one on the west and the other on the east of the 

 narrow strait which separates the island from the continent, 

 appearing above the amygdaloid which forms the base of the 

 hills ; and therefore, leading us to conclude, that its ejec- 

 tion has been subsequent to that of the amygdaloid. 



The alteration which the eruption of the basalt has pro- 

 duced on the masses through which it has been forced, by 

 rendering the two rocks at the point of contact similar, 

 and as if passing into each other by a gradual transition, 

 are sufficiently obvious, but at the same time, the two varie- 

 ties are as distinct as any of the projected series in general 



