104 SPOLIA ZEYLANICA. 
of kaolin or china clay (with, perhaps, some aluminium 
hydrates) and iron oxides. The sand grains themselves are 
thickly coated with iron oxide, which often requires acid to 
remove it. The surface of the grains beneath this red coating 
are peculiar. Under the high power of the microscope, and by 
reflected light, they are seen to be polished and covered with 
the tiniest little pits imaginable. Moreover, sharp angles are 
generally wanting. The grains are all small, and seldom 
exceed one-sixteenth of an inch in diameter, while the majority 
are smaller. Most of the grains are quartz, but other minerals, 
such as spinel, zircon, ilmenite, &c., occur sparingly. 
Rounded grains of quartz, like these, do not exist in the 
rocks from which they were certainly derived ; so that, like 
pebbles, they must have acquired their roundness by a process 
of attrition. 
If you examine the sand of rivers or of the sea, you will find 
that itis sharp. Thereason for this is that the grains have 
been protected from excessive abrasion by the water, which 
acts as a cushion or fender between the solid particles. 
In wind-blown sand, however, the grains are typically 
rounded, while the sand of most deserts includes a varying 
proportion of millet seed, or practically spherical particles. 
The reasons for these differences are given by Hatch thus :-— 
~“ Under wind action the rolling motion is predominant, 
while sliding is subordinate ; secondly, when a sand grain 
moves in air, the whole weight is effective in producing 
friction, while in water only part comes into play.”’* 
So that while sand deposited by water is sharp, that blown 
by the wind is rounded. Another distinctive feature of sands 
is the presence or absence of mica. This mineral is nearly 
always present in aqueous sands, but practically never in 
wind-blown sands, though exceptional cases, of course, occur. 
Acertain proportion of millet seed grains is to be found in the 
red earth, while all the quartz particles are characteristically 
rounded. These facts taken together with the absence of 
mica point very decidedly to an eolian origin of the red earth ; 
but there are difficulties. The wind in transporting material 
* Hatch and Rastell: ‘“‘ The Petrology of the Sedimentary Rocks,” 
London, 1913, p. 54. 


