562 Notices of Memoirs—Desert Conditions in the Trias. 
bedding the sands are often contorted at the bottom of the basin as 
though the weight of the sands above had caused a slide such as we 
have described as taking place in sand-hills. 
It is difficult to describe the structure of the sandstones; there are 
no geological terms exactly suitable. They have the appearance of 
a tumbled series of eroded lenticles. Anyone who has had the 
misfortune to map them knows how the beds thicken and thin out 
promiscuously, and how difficult it is to find a datum-line which will 
be of service in correlation. Even our late member and founder, 
Mr. Morton, whose knowledge of our local rocks was so intimate and 
extensive, would never give an opinion as to the horizon of a piece of 
Trias sandstone from a hand specimen. The only approaches to 
satisfactory datum -lines we possess are the two horizons where 
pebbles occur, and even these are only of service locally, as they are 
limited in extent. In sections cut through dunes in the desert 
between Ismailia and. Kasassin I have seen beds very much 
resembling those I have attempted to describe above. 
The marginal beds of the Trias area are almost without exception 
the Keuper Marls. They are found also covering the sandstones in 
the middle of the area, and sometimes they are intercalated with beds 
of sandstone. They are not strictly marls, but are composed almost 
exclusively of exceedingly fine and angular quartz dust. It has been 
suggested that they have been laid down in a lake, or a series of lakes, 
and the even bedding which they show is given as a proof. But 
bedding even more perfect may be seen in volcanic dust deposited on 
the land, and, further, Mr. T. O. Bosworth has shown that when the 
Keuper Marl rests against a sloping cliff the bedding slopes with the 
surface of the ground. ‘This is quite unlike the conditions we should 
expect to find if it had been of subaqueous origin. Lakes of the 
desert type are found as local phenomena in the Keuper Marls, and 
have been referred to in a previous part of this paper. The marls 
may represent the smallest tailings of wind-carried material. 
The sifting action of wind is everywhere evident in the Trias. 
Besides the horizontal grading already mentioned which characterizes 
the deposits broadly as a whole, we find that locally the same sifting 
influences have been at work. Sands of exactly the same dimensions 
occur as lenticular patches, a few feet or several yards in length. 
These are associated with other lenticles of larger or smaller grain, 
but in the same patch there is no admixture of large and small sizes. 
In our neighbourhood they are best seen at the top of the Bunter, 
immediately underlying the Keuper basement bed. The perfection of 
sifting into sizes shown at Bidston and at Scarth Hill is marvellous, 
but in other places and at other horizons the some features may be 
observed. 
The concentration of pebbles from river deposits is also paralleled in 
the Trias. The pebble beds of the Midlands, although originally of 
fluviatile origin, do not exhibit the characteristics of river action. 
The individual pebbles show no orientation in the arrangement of 
their longer axes, but are wedged together in a tumbled mass as if 
they had dropped into their present situations by the removal of 
material about them. The insecurity of their positions is evidenced 
