84 Reports and Proceedings. 



Prof. Tennant made some remarks on the gratitude due to the first geological 

 observers in South Africa, who had, by the discovery of the Diamond-fields, so 

 materially added to the wealth of the colony. During the last month he had seen 

 no less than ;i{!^ioo,ooo Vorth of diamonds brought over by three persons. At 

 present some twenty thousand persons were employed in the fields, and the 

 diamonds were equal to any in the world. The trap-rocks in South Africa were 

 as various in their character as those in this country, and this variety might well 

 lead to speculation as to their origin. 



3. " On the Mud-craters and Geological Structure of the Mekrau 

 Ooast." By Lieut. A. W. Stiffe, F.E.A.S. Communicated by Prof. 

 Eamsay, F.R.S., V.P.G.S. 



The coast of Mekran, extending from near the western frontier of 

 India to the mouth of the Persian Gulf, was stated by the author to 

 be a nearly rainless district, consisting of clay plains with pre- 

 cipitous tabular hills, the former veined here and there with crystal- 

 line gypsum, the latter composed of clay capped and sometimes 

 interstratified with coarse, friable, fossiliferous, calcareous strata, 

 from 5 to 30 feet thick, supposed to be of Miocene age, and all 

 horizontal or nearly so, except at the extreme east and west, where 

 the strata are inclined at an angle of from 40° to 60°. Along the 

 coast there are no distinct traces of volcanic action, but on the 

 north coast of the Persian Gulf a similar formation has been much 

 disturbed by the protrusion of recent volcanic material ; near Jashak 

 to the west there is a hot mineral spring, and near Karachi there 

 are springs of pure hot water. The author described the mode in 

 which denudation is effected in this region by occasional heavy 

 rains, and by the constant action of the sea upon the coast, and then 

 noticed the occurrence, within a few miles of the shore, of numerous 

 peculiar mud-craters, forming hills varying in height from 20 to 

 300 or 400 feet above the plain, of a regular conical form, with 

 truncated tops, and the sides sloping at an angle of about 40°. The 

 summits of these hills present a circular cup with a narrow border, 

 filled with semifluid mud, which occasionally flows slowly over the 

 margin of the crater. The author considered that the conical hills 

 have been formed solely by these overflows. He believed that a 

 small shoal occurring off the coast near Jashak might be produced 

 by one of these craters, and was inclined to ascribe their existence 

 to hydrostatic pressure rather than to A^olcanic action, especially as, 

 by the concurrent testimony of several natives, the discharge from 

 the craters is greater during spring tides. The thickness of the 

 clay forming the plain is probably very considerable ; it extends for 

 some miles from the shore, sinking gradually to 20 or 30 fathoms, 

 when there is a sudden and often precipitous descent to a depth 

 of 300 or 400 fathoms. The author suggested that, since the de- 

 position of the Miocene beds, the great submarine cliff may have 

 been raised above the sea ; that the land was then depressed to near 

 its present level, causing the removal of the beds to the present 

 coast line, and that a further depression followed by upheaval gave 

 origin to the inland cliffs. Evidence of the last depression is fur- 

 nished by the presence of borings of lithodomous mollusca in the 

 cliffs considerably above the present sea-level. 



