168 SPOLIA ZEYLANICA. 
going’ that the Arabs are obliged to fit leather shoes to 
their camels in order to traverse this unhospitable ground 
(plate II., fig. 2, and plate III.). Mushroom-shaped rocks, 
which are common features of some arid regions, are hardly 
represented in this country ; but hard stones polished by the 
attrition of the blowing sands are by no means rare (plate 
IV., figs. 2 and 3). Speaking of deserts in general Prof. James 
Geikie says :— 
So effective is the action of the sun and wind that the whole 
surface of a rainless region is gradually denuded and lowered, the 
loose materials continually travelling onwards to the borders of 
the desert. The sands of continental dune-lands, therefore, have 
no necessary connection with abandoned sea floors. It is true 
that within certain desert areas there exist lakes and inland seas 
that are gradually drying up. In the great desert of Gobi, for 
example, lakes occur which have obviously at one time been 
considerably more extensive. So again in the Aralo-Caspian 
depression, abundant sand hills are scattered over wide areas, 
which were certainly under water at quite a recent geological 
date. Within such tracts, therefore, after the water had dis- 
appeared, much loose sand was doubtless already prepared for 
the direct action of the wind. But in the case of extensive 
deserts, such as those of North Africa, Central Arabia, &c., the 
sand has been derived almost wholly from the sub-disintegration 
of rock.* 
During the long dry season the heat is intense, and not a few 
stones which have offered a resistant face to the ravages of 
time have been burnt brown or black by continued exposure 
to the sun (plate IV., fig. 2). But sun-baked stones are 
much more common in or near the river beds than on this 
practically waterless tract. Though heat is certainly one 
factor in the process of tanning, moisture is undoubtedly 
another, and the present writer remembers his astonishment 
at seeing certain rocks near the Nile, on the Sudan border 
land, which were markedly more sun burnt on the sides 
which obtained most shade than on those which were subjected 
to the glare of the sun all day. 
Disintegration through the action of solar heat is the most 
potent form of weathering in the desert tracts (a), (6), and (c) 
with which we are dealing ; wind erosion has accomplished 
comparatively little. Its effects are, however, to be seen, and 


*** Mountains : their Origin, Growth, and Decay,” 1913. 
