1869.] CLAEK INDIAN BASALT DYKES. 165 



right angles to the face, or plane of cooling, of the dyke. Almost 

 all are subdivided by planes parallel to that plane, into bands ; and in 

 most of the larger dykes the interior bands are more or less concre- 

 tionary. All are much jointed, evidently in cooling, and are fissured, 

 or readily fissile, to great minuteness. The fragments, almost always 

 freely scattered about along the course of a dyke, are commonly flat 

 triangular prisms, having two faces of unequal breadth, but much 

 broader than the third, and consequently containing a very acute 

 angle. The material is very dense and cHuking, and, so far as I have 

 observed, is never amygdaloidal, nor, excepting in a very few cases, 

 vesicular. This is remarkable, since four-fifths of the rock through 

 which the dykes pass is highly amygdaloidal. 



These dykes have been traced through every rock from beneath 

 the sea-level up to the crests of the Ghauts, and nearly to the sum- 

 mit, geologically, of the highest outliers of the Concan. They are 

 found 4000 feet above the sea upon Hurreechunder, and at full 

 3000 feet between the Malseje and Joodhun. They have not 

 hitherto been observed in the basalt capping of Beema- Hunker ; and 

 therefore their relation to that bed is uncertain. 



Except in a few and local instances, I have been unable to detect 

 anything like the efiects of secondary fusion in the minute structure of 

 the trap beds cleft even by the larger dykes, though these beds, being 

 amygdaloidal, might be expected to show some traces, even mecha- 

 nical, of the great heat to which the dyke must have exposed them, 

 whether this occurred before or after the deposition of the zeolite. 

 Viewed in mass, it is, however, evident that the trap on either side of 

 each dyke has, for some distance, been hardened or rendered tougher 

 by its heat, and has thus been enabled to oppose special resistance 

 to the eroding forces. Hence when the dykes traverse a trap plain, 

 their course is usually indicated by long and lofty, though often 

 narrow ridges of rock, of which the dyke forms the axis, and which 

 from their superior toughness have resisted the general removal of 

 the surface. The actual course of the dyke itself is usually marked 

 by a depression, the prismatic structure allowing it, however hard 

 the rock, to be removed in fragments. In a very few cases planes 

 of fissure or cleavage are observed in the trap parallel to a dyke, and 

 apparently due to the secondary fusion of the trap by the dyke. 



The plane of separation between the dyke and its containing rock 

 is, commonly, well defined. There is no fissure, but there is no adhe- 

 sion. Sometimes branch dykes come off", and almost always at an 

 acute angle. 



The ordinary structure of these dykes seems to be homogeneous. 

 The grain is usually fine, though now and then coarse and open. 

 Occasionally they contain crystals of a honey- coloured mineral. 

 Some are pitted superficially, where exposed to the weather, no 

 doubt by the decomposition and removal of this mineral. One dyke 

 near Callian affects the magnetic needle considerably ; and a few 

 others do so in a less degree. One or two of the Callian dykes are 

 contained between a sort of selvage made up of plates of about one- 

 eighth of an inch thick, but with the planes of separation of each 



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