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Geological Desiderata. 



[Afril 



attention has hitherto been paid to these important subjects by Indian 

 geologists. 



3. — Observations tending to elucidate the causes and conditions of 

 the modifications of structure and morphic changes, remarkable in even 

 continuous, and apparently homogeneous masses of both normal and 

 abnoi-mal rocks; and e^ecially obvious near their line of contact with 

 rocks of a different mineral character. 



4. — Similar observations on basaltic dykes, and the rocks they pass 

 through; noting the various amount of disturbance and alteration, with 

 reference to the magnitude of the djke. It will not escape the atten- 

 tion of observers that the basalt itself has been in some measure reci- 

 procally acted upon by the rocks it penetrates. For instance,— the 

 basalt passing through the clay-slate of the eastern ranges of the Ceded 

 Districts, which is frequently converted into flinty slate or becomes 

 crystalline and hornblendic, acquires a distinctly slaty structure, and 

 so much modified in mineral character as scarcely to be distinguished 

 from the altered clay-slate in its vicinity. In thin veins and near the 

 edges of large dykes the basalt becomes compact; but towards the cen- 

 tre, where more slowly cooled, it is almost invariably of a crystalline, 

 and sometimes of a porphyritic structure. Where basaltic dykes abound 

 a general tendency to crystalline and metallic development will be re- 

 marked ; as well, perhaps, as an increase in the deposition of saline 

 and calcareous matter, — the sub-carbonate, muriate, and sulphate of 

 soda. Those extensive depositions of nodular, tufaceous, travertine- 

 like, and friable cavboBate of lime, — the well known ku7ikar of India,— 

 appear to have their origin from springs rising up from nearly vertical 

 fissures in the subjacent rocks, which are invariably more numerous in 

 the vicinity of great basaltic dykes, and have been caused, possibly, by 

 the same disruptive forces that opened a vent to the basaltic matter 

 with which many of the greater cracks are filled. A series of extensive 

 observations incline me to the opinion that this basalt, and not granite, 

 as generally thought by Indian geologists, is the lowest rock found in 

 peninsular India, and that its development h;is a much greater influiince 

 over the direction and altitude of mountain chains than the usually 

 supposed granite ; which, though frequently observed with the uplifted 

 schists resting on its shoulders, will, where opportunities of examina- 

 tion occur, be found to be merely an intermediate rock between the 

 basalt and the normal rocks it underlies. It must not be forgotten in 

 making comparative observations on the direction of the basaltic dykes, 

 and on that of the mountain chain near which they occur, that we are 

 most frequently doing so on branches merely from the main dyke, which 

 diverge from it at various angles with its course, while the main dyke 



