Iixx PROCEEDINGS OF THE GEOLOGICAL sociEry. | May 1902, 
which our great continental formations, such as the Torridonian, 
the Old Red Sandstone, and the Trias, have originated. 
~ Deserts and areas of closed drainage, like ocean-basins, are the 
receptacles for the detritus worn from the surrounding lands; but 
the phenomena of denudation and deposition are widely different in 
the two cases. Dry weathering, torrential rains, and wind are the 
three most potent agents in arid climates, and by their combined 
action inland rock-basins of vast extent become filled with enormous 
accumulations of detrital material. ‘The violent extremes of tempe- 
rature not only detach fragments from every exposed surface, but 
often loosen the constituents of crystalline rocks, so that at the 
slightest touch a piece of apparently solid granite will crumble into 
sand, leaving the felspars as fresh as when they formed a part of 
the original rock. 
Cloud-bursts follow at intervals of months or years, and the vast 
accumulations of detritus of all kinds, both large and small, mingling 
with the waters, are swept along in one tumultuous flood and spread 
out over the plains in extensive flat fans. The action is so sudden 
and catastrophic, that there is no time for that careful sorting of 
materials according to size and specific gravity, which takes place 
during the formation of marine deposits. Sand-dunes wander over 
the plains, and cover up the coarse breccias and conglomerates formed 
by the torrential rains, and then follows another cloud-burst. 
Wind sweeps the deserts free from dust, and spreads it far and 
wide over the surrounding districts to form, under favourable 
circumstances, the thick beds of loess with which Richthofen has 
familiarized us. 
But areas of closed drainage are not entirely desert. The rainfall 
may be sufficient to form temporary or permanent streams ending 
in salt-marshes, salt-lakes, or inland seas which may contain the 
dwarfed relics of a marine fauna, rich in individuals but poor in 
species, and give rise to the formation of beds of rock-salt and 
gypsum, such as those with which the geologist is familiar. 
Thus geographical and oceanographical researches during the 
latter half of the century, combined with a study of the stratified 
rocks, have brought into prominence the contrast between conti- 
nental and marine formations, and have familiarized us with the 
different ‘ facies’ of these two strongly contrasted types. We can 
now picture to ourselves the onward sweep of the sedimentary 
zones, aS the sea slowly advances upon the land and deposits its 
sediments upon a plain of marine denudation, and the gradual dis- 
