16 
LOMAS : ON THE ORIGIN OF THE TRIAS. 
explains one of the features which strikes an observer in visiting 
a desert region, where he finds great stretches of pebbles of 
all sizes strewing the ground, while in the distance he may see 
the dunes of sand composed of materials which were formerly 
mixed with the pebbles. 
When deposits such as these are worked up and re-worked 
up by wind action year after year, there is a tendency towards 
a concentration of particles of one size at one place. We shall 
see that in other respects concentration is the dominant feature 
of desert areas. 
Another way in which the desert shows itseK as an area 
of concentration is in the materials brought down by rivers in 
solution. Evaporation may take place during transit, and 
the sand through which the streams flow may be coated with 
salts. In flashes or shallow pools held up by the irregular 
disposition of sand dunes, water may remain for a longer period, 
even through the dry seasons, and then it becomes strongly 
charged with salts, which may crystallise out as common salt, 
gypsum, iron pan, carbonate of lime, and other substances. 
Mud and drifted vegetation carried down by the streams may 
be deposited at the bottom of these flashes, forming a puddle, 
and preventing the escape of water below. On drying, we 
have left a mud, strongly charged with salts, which cracks 
on desiccation, and this may take the imprints of animals 
which come to drink and impressions made by rain drops and 
runlets of water. Sand blowing into these shallow pools will 
bury the lower deposits, and preserve there the markings im- 
pressed on them. In the deserts of South Africa we often 
find white nodules in the sand consisting of carbonate of lime 
cementing grains of sand. These vary in size from a marble 
to gigantic masses. Sometimes they form continuous layers, 
and I have seen them 10, 20, or 30 feet thick. The carbonate 
of lime is precipitated out of solution by loss of carbonic acid, 
and after deposition it is not easily removed from the sand 
grains it coats. Hence carbonate of lime concentrates on land 
rather than in pools. 
A third way in which concentration is seen in deserts is 
shown by the manner in which organisms tend to cluster round 
