part 1 .] Feistmantel: Cretaceous genus Omphalia, near Namcio Lake. 21 
the province, from the Ran of Catch to the Bah&walpur territory. The other tract 
extends northward or north-north-east, also from the neighbourhood of the Ran, and was 
crossed east of Balmir, and again between Jodhpur and Pokran. It appears to extend 
towards Bikariir. 
The hills in the western tract are arranged in regular parallel or nearly parallel ridges 
running nearly north-east and south-west to the southward near Umarkot, while further 
north, towards Rohri, they have a direction from south-south-west to north-north-east. The 
ridges frequently end abruptly with a steep slope to the north-east. 
Elsewhere the sandhills are not arranged in parallel ridges, but are more or less thickly 
scattered over the surface, and have always a steep slope to the north-east and a long 
gentle slope to the south-west. It is evident that the sand has been transported and 
deposited by the strong south-west winds of the hot season—May, June, and July. The 
origin of the parallel ridges is much more obscure, but there can be very little doubt of 
their being due to the south-west wind*. 
The sand consists chiefly of rounded quartz grains, felspar, hornblend and one or two other 
minerals being also present in small quantities. A portion of it may be derived from the 
Indus, but a far larger proportion must be due to some other source. Many of the sand-hills 
are evidently of great antiquity ; despite the small rainfall of the desert region, they show 
signs of considerable denudation in parts, and are cut into deep ravines by the action 
of water. 
It is highly probable that the Ran of Cutch is an old inlet of the sea, which has been 
filled up by the sediment brought in by the Luni and other rivers. The presence of a marine 
mollusk, living in the salt-lakes north of Umarkot, proves that this inlet extended far up 
the Indus valley, and the great saltness of the soil, both in the Thar and in the Luni valley, 
suggest the probability of the sea having extended in both directions. The shore of this 
great inlet may easily have supplied the blown sand which now covers so large an area of 
the desert, and the distribution of the sand-hills nearly coincides with what might be expect¬ 
ed if the sands were derived from such a source. 
Throughout the greater portion of the desert there is no evidence of marine denudation. 
Nothing of the kind is seen neer Balmir, and the scarps near Jesalmfr are evidently due to 
subaerial action, and quite different in appearance from sea cliffs, each being formed by the 
outcrop of a hard bed. It therefore appears probable that the central region of the desert 
was above the sea, forming either a promontory or an island, whilst the Ran of Cutch, the 
Indus valley, and portions of the Luni valley were under water. 
On the occurrence of the cretaceous genus Omphalia near Namcho Lake, 
Tibet, about 75 miles north of Lhassa. By Ottokar Eeistmantel, m. d., Geological 
Survey of India. 
Last year (1876) the Geological Survey received some fossils from Captain Trotter, 
which were collected by one of the Paudits attached to the Trigonometrical Survey of India 
on his route from Ladak (Leh) to Lhassa.f 
The most peculiar amongst them appeared to Mr. W T. Blanford, who first received 
this collection, and subsequently to me, some Turritella-Yike forms, which were especially 
characterised by two well-marked, prominent ribs in each whorl (in the whole height of the 
* See for further details J. A. S. B., 1876, XLV, Ft. II, pp 92,67, &c, 
f See sketch-map in Geographical Magazine, June 1876. 
