15 
ON THE OEIGIX OF THE TRIAS. 
BY J. LOMAS, A.R.C.S., F.G.S. 
It is becoming more generally recognised day by day that 
the Triassic Rocks of Great Britain can best be explained by 
assuming that desert conditions existed during their formation. 
In order to test this it may be well to examine briefly the 
conditions now existing in deserts and the activities now at 
work in those regions. While desiccation is one of the control- 
ling factors in the deserts of to-day, we find none which are 
absolutely free from water. Deserts may exist with a com- 
paratively large rainfall, provided it falls in a limited time, is 
quickly shed, and not stored up by glaciers, soils, or lakes. 
Thus streams which exist in the rainy season soon become 
dry, and for the rest of the year they are not functional. In 
South Africa we constantly come across deep channels excavated 
by streams, their beds covered with large stones which could 
only be carried by torrents. During the dry season sands 
blow into the hollows and completely obliterate aU traces of 
the stream courses. In the next rainy season the streams 
may or may not find their old channels, and thus by constant 
excavation and in-fiUing we get deposits covering the country, 
consisting mainly of blown sands with patches of gravel marking 
the sites of former streams. 
Wind acting on deposits like those just described sweeps 
away the sand to longer or shorter distances to form heaps 
or dunes. The finer particles will come to rest at one place 
and the coarser at another. In this way we get a more or less 
perfect sifting into sizes. I have examined many samples of 
sand from the Sahara and other deserts, and find this sifting 
to be one of the chief characteristics of blown sand. In one 
place I was fortunate enough to get specimens of beds in vertical 
order, and found that while the grains were uniform in size 
for the same horizon, they differed very much from bed to bed. 
The stones too heavy to be moved by wind tend to drop down 
to a lower and lower level, and form a stratum composed wholly 
of large stones, with little or no sand intermixed. This 
