18 
LOMAS : ON THE ORIGIN OF THE TRIAS. 
The striking feature of the Upper Bunter is the way in 
which the various horizons are characterised by even sized 
grains. In some beds it looks as if the material had been put 
through a sieve or a series of sieves of different meshes. A bed 
may preserve an even grain for a few inches only, and be suc- 
ceeded by one of different size, but uniform for the same horizon. 
In no deposit laid down under water could this feature be so 
perfectly developed. 
Coming now to the Keuper, we find the basement beds are 
characterised by the occurrence of pebbles and rolled cla}^ nodules, 
and a thin band of marl, at least in the north, usually intervenes 
between the Keuper and Bunter. In places, as at Alton in 
Staffordshire, concentration of pebbles has taken place and wind 
etched pebbles are not uncommon. 
The Lower Keuper, I believe, marks a pluvial phase following 
desert conditions. Above this the Keuper sandstone is character- 
ised by an increasing frequency of marl bands. These are never 
of very great extent, and are alwaj^s half lenses with the convex 
surface downwards. They may represent the flashes and shallow 
pools of the desert or semi-desert region, and it is on their shores 
we find the footprints of reptiles, Estheria, and other organisms, 
and pseudomorphs of rock salt. The last phase of the Keuper is 
represented by the Keuper Marls. It is a misnomer to call them 
Marls, as clay enters but little into their composition, except in a 
few localities. The mass of the rock is composed of exceedingly 
fine, splintery, angular grains of quartz which seldom exceed 
to 37^00 diameter. Large pools or lakes existed at this time 
with the concomitant beds of salt, gypsum, &c., and footprints, 
Estheria, and drifted plant remains are common. The Keuper 
Marls may represent the material accumulated by the feeblest 
currents of wind, the fragments blown furthest, and as such may 
be compared with the Loess of Eastern Europe. 
Taken as a whole, we may regard the Trias formation as 
made up of a series of lenticular beds, irregular in their dis- 
position and piled up mostly by wind action, but before they 
can be properly read we must know a great deal more about 
deserts, not only what they are, but what they do. 
