170 SPOLIA ZEYLANICA. 
facets; some have more, many have less. The example 
figured in plate V. has one wind-cut face—the curved face ; the 
plain surfaces are the result of jointing in the original rock. 
A peculiarity about faceted stones is that while they are 
characteristically desert products, they are often thinly 
distributed and are only locally abundant. In (a) desert 
faceted stones are scarce, but polished pebbles are not un- 
common, while a fair proportion of stone age artifacts, in 
which the tract is peculiarly rich, bear a high lustre. 
Besides these wind-cut and wind-polished stones some 
cherts occur with curious spongy surfaces ; indeed not a few 
of the stone artifacts which were gathered on these sites 
showed this form of weathering (plate IV., fig. 1). It can 
hardly be doubted that the weathered surface is the result of 
exposure of the artifacts since the days of their manufacture. 
Many of the stone tools of the Lybian and Egyptian deserts 
are affected in a precisely similar manner, and it seems 
probable that the spongy exterior of these stones points to 
exposure continued over long periods of time, and may there- 
fore be taken as a fair criterion of antiquity. It should be 
noted, however, that not all varieties of stone are subject to 
this form of decay, and therefore the absence of a spongy 
surface is no argument in favour of a recent production. In 
the absence of vegetation, the want of water, the characteristic 
features of erosion and sun baking, the tracts (a), (b), and (c) 
are typical deserts conforming more to the rocky type than to 
the sandy variety. They are peculiar in that they are cireum- 
scribed and abrupt in their occurrence. The exact mode of 
their origin is not altogether clear, but the following facts are 
suggestive :— 
(i.) They are not far from rivers which appear to have 
dwindled in volume since their valleys were 
excavated. 
(ii.) Their surfaces are extremely unfertile, for although 
of lower altitude than the surrounding forest 
country they support no vegetation. 
(iii.) There is no reason to suppose that they are chemi- 
cally unfertile. 
