The Laccolites of Cutch. 129 



insists also on the proofs of close genetic relations between the 

 gabbros and ' granophyres,' and of their practical contemporaneity in 

 the various districts in which these phenomena have been noticed. 



2. ' The Laccolites of Cutch and their Relations to the other 

 Igneous Masses of the District.' By the Rev. J. F. Blake, M.A., 

 F.G.S. 



The author has observed thirty-two domes of various kinds in 

 Cutch, distributed as follows : — (i) those connected with the northern 

 islands ; (ii) those of Wagir ; and (iii) those along the northern edge of 

 the mainland. They are divisible into four classes : (a) those which 

 are so elongated on the line joining adjacent ones that they seem to 

 be mere modifications of anticlinals, though the supposed anticline 

 is not really continuous ; (5) those which lie in a line, but are not 

 elongated in that direction, and often in no other ; (c) those which 

 are related to a fault, which cuts them in half; and (cl) those which 

 are not in any particular relation to each other, or to any other 

 stratigraphical feature. 



The domes vary in degree of perfection : some are irregular, 

 while some have the strata running in concentric circles, the outer 

 and newer strata dipping away from the inner and older. In no 

 less than ten of the thirty-two domes igneous bosses are found 

 occupying the centre, and these are distributed amongst all of the 

 above classes. The author gives reasons for maintaining that the 

 domes are the results of intrusion of igneous rocks in the form of 

 laccolites, and are not anticlinal folds which have afterwards been 

 affected by cross-folds. The domes are contrasted with igneous 

 peaks which occur in abundance in a different part of the area, 

 usually at a higher horizon of the strata and at a higher level above 

 sea. These are probably volcanic pipes through which the lava was 

 forced and extruded at the surface. 



The author compares the rocks of the bosses with those of the 

 dykes and flows. Both are principally perfectly fresh dolerites, but 

 the former are distinguished by the presence of intergrowths of 

 micropegmatite as the last stage of consolidation, as in the ' Konga 

 diabases/ There is also among them a felsite-breccia with micro- 

 pegmatite developed in the cracks. 



He considers that nearly all the igneous rocks of Cutch have been 

 derived from a single magma, which in a solid condition must have 

 contained large crystals of augite, olivine, and ilmenite in a ground- 

 mass of lime-felspars, and have been throughout of a basic character. 

 Such a magma originated in more than one centre. One was possibly 

 not far from the Sindree basin, whence lines of weakness diverged. 

 Along these, owing to the thickness of the strata, there was no 

 extrusion at the surface, and laccolite- domes were formed. Where 

 the rock reached higher levels, it spread out into sheets between the 

 domes and aided in the production of synclinals. Another centre 

 was west of Bhuj, where the rock reached the surface without 

 materially disturbing the sedimentary rocks, and formed the so-called 



Phil. Mag. S. 5. Vol. 44. No. 266. July 1897. K 



