12 EEV. J. F. BLAKE ON THE LACCOLITES OP CUTCH, [Feb. 1 898, 



2, The Laccolites of Cutch and their Relations to the other Igneous 

 Masses of the Disteict. By the Rev. J. F. Blake, M.A., F.G.S. 

 (Read May 26th, 1897.) 



[Abstract.] 



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 ; (6) those which lie in a line, but 

 are not eloDgated in that direction, and often in no other ; (c) those 

 which are related to a fault, which cuts them iu half ; and {d) 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 cr^^stals 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 



