TRANSACTIONS OF SECTION C. 675 



The working up of such deposits by wind results iu the concentration of the larger 

 stones, while the smaller saud grains are drifted to form dunes. 



There is also a tendency for sand grains of a certain size to concentrate in one 

 place, owing to variations in the carrying power of the wind. Beds in vertical 

 order show this feature in a marked degree. 



It would appear that one of the dominant characters of desert regions is 

 concentration, and this is shown not only in the sifting of materials, but in many 

 other ways. Thus matter brought down by rivers in solution may be deposited 

 round sand grains even during transit, while * flashes ' and shallow pools fed by 

 streams become strongly charged with salts, such as common salt, gypsum, iron- 

 pan, carbonate of lime, and other substances. Mud and drifted vegetation puddles 

 these ' flashes ' and prevents the escape of water. On drying, a mud is left strongly 

 charged with salts, marked by desiccation cracks, and covered with the imprints 

 of animals which have come to drink. In many cases the muds of these shallow 

 pools swarm with living Edheria. 



In many desert localities we find white nodules, consisting of carbonate of 

 lime cementing sand grains. These vary in size from a marble to continuous 

 layers of great extent and 30 feet thick. Carbonate of lime tends to concentrate on 

 land rather than in pools. A third way in which concentration shows itself is 

 in the clusterbg of organisms round the pools where water is more or less 

 permanent. 



Applying these principles to the Trias, the existence of a great spread of river- 

 borne material, the origin and disposition of which had been dealt with by Pro- 

 fessor Bomiey, may be assumed. 



The pebble beds of the Midlands may be the coarse concentrates of such a 

 deposit, while the pebble beds' of Lancashire and Cheshire may represent com- 

 paratively unaltered fluviatile deposits, augmented and thickened by the addition 

 of the finer material from the Midlands. 



The Upper Bunter is characterised by the perfect sifting into sizes which the 

 grains of any one bed show. 



The Keuper basement-beds contain pebbles and rolled clay nodules, con- 

 centrated in places, chiefly in the Midlands. 



Some of the pebbles are dreikayiter. The Lower Keuper may represent a 

 pluvial phase, following the arid conditions of the Bunter. 



The Keuper Sandstone contains inclusions of marl with increasing frequency 

 as we ascend from the base. The marl bands are not of very great extent, and 

 are always plano-convex lenses with the convex surface downwards. These may 

 represent the sites of dried-up pools, and it is here that we find footprints, Estheria, 

 and other organisms, pseudomorphs of rock salt, &c. 



The highest beds represented by the Keuper Marls are composed almost 

 entirely of exceedingly fine splintery quartz dust, the particles not often exceeding 

 yj-^ inch diameter. Large pools existed at this time with their concomitant beds 

 of salt, gypsum, &c., and Estheria and drifted plants are common. 



The Keuper Marl may represent the finest residuum of wind-sifting, and may be 

 compared with the loess of Eastern Europe. 



(iii) By T. H. Holland, F.R S. 



Mr. Holland referred to certain phenomena in the Eajputana desert that sup- 

 ported Mr. Lomas's views with regard to the processes of concentration in arid 

 regions. He referred to the separation of the finer angular sands (which were 

 carried bodily in the wind, and became deposited in the north-eastern area when 

 the force of the wind was diminished) from the rounded grains formed by the 

 rolling of grains too heavy to be carried by the wind. With the finer material in 

 the neighbourhood of Bikanir, Mr. La Touche had described the occurrence of 

 undamaged tests of Foraminifera, which must have been carried in the wind some 

 five hundred miles from the Kathiawar coast. Mr. Holland also described the 

 occurrence of the silt bodies filling in hollows in the Archaean surface, having a 

 general plano-convex lens shape, and being charged with salt, beds of gypsum, and 



