Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/259

1869.] CLARK-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 concretionary. 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 clinking, 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 summit, 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-Sunker; 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 effects 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 mechanical, 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 adhesion. 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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