Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/223

ROGERS — WESTERN INDIA. 123 laterite is the same in age and composition as the laterite of western India. Their difference has already been pointed out by Newbold and Aytoun; and that in Madras may be presumed, for the following reasons, to be of a much later date than that of Bombay, probably only a lateritic or ferruginous conglomerate. The laterite of Bombay overlies the great trap formation at all elevations of from less than a hundred feet above the level of the sea in the country round the Gulf of Cambay to 5000 or 6000 feet above it on the highest Ghaut-level. In connexion with the Turkeysur nummulitic rocks, Mr. Wynne, in his Report on the Geology of the Surat Collectorate, speaks as follows: —

"A low range of hills rises near this town and stretches southwards towards the Taptee; they are formed of ferruginous or lateritic beds intercalated between agate conglomerates, and, having a low dip to the west, they pass beneath the limestone just mentioned [grey nummulitic], which, however, is traceable along their flank and reappears in the Taptee river at the end of the range, being let down by a fault to a lower level, but preserving its westerly dip, and seen to be overlain again by another band of laterite."

Here it appears that laterite is both below and above Nummulitic limestone. There can thus be no doubt that this laterite belongs to the Lower Eocene. The rocks of the Nummulitic series near Gulla on the Taptee, at the distance of a few miles south of Turkeysur, are reported by Mr. Wynne to overlie those of the latter place, and are thus a little higher in the Eocene. The laterite in the neighbourhood of Turbhan, to the south-east of Surat, is described in the same report as compact and brecciated, and as being of a very similar character to that just mentioned. There seems to be no doubt that this laterite is precisely similar in mineral character to that of the Deccan, which caps the western Ghauts or Syhadree range, Mahableshwur, &c, frequently at the top of precipitous scarps thousands of feet in height above the level of the low country of Goozerat. There is no apparent reason for supposing that the laterites of the high and low levels are referable to at all widely separated geological ages, the evidence of the agate conglomerates near Turkeysur, intercalated between lateritic beds, being the only proof of the rock having been deposited at different times.

Mr. Blanford, in reporting on the age of the traps, admits that denudation must have taken place to an immense extent after the Nummulitic rocks were deposited; and the rare occurrence of laterite in the low country may probably be attributed to this. Subsequent upheaval, too, is proved by the position of the lateritic beds to the south of Gogo; these have been broken up from below, and lie as they were thrown over on the flanks of some of the hills. This might account for the disruption of the Island of Perim from the mainland; but only a much more violent action would have separated the laterites of the high and low levels, supposing them to have been deposited under similar geological circumstances. This rock, again, appears at precisely the same level on the opposite sides of valleys in the Concan and Deccan, giving ample proof of denuda-