Dr Elemmg's Remarks on the Modern Strata. 119 
phere, or the attrition of rivers or the sea,-— are not all equally 
favourable for the growth of plants. When the rocks, which 
have been disintegrated, consist nearly of pure quartz, and the 
result is a siliceous sand, the soil is not only unfit for the growth 
of vegetables, but, from its want of cohesion, is not stationary. 
The wind regulates its condition. The strata of this division, 
in reference to their origin and position, may be divided into 
two groups. The first will comprehend the inlcmd sand-drifts^ 
forming deserts^ striking examples of which occur in Africa and 
Asia. To the second, will belong the shore sand-dr^ts^ form- 
ing downs, of which our own country presents several well 
marked illustrations. The materials, in the latter case, chiefly 
consist of sand, derived from granitic or sandstone rocks, and 
portions of comminuted shells. Shore sand-drift spreads over 
the soil, and destroys its vegetable cover. In some places, I 
have seen it forming a thick bed over forest and moss peat ; the 
latter exhibiting unequivocal symptoms of rapid decomposition. 
3. Detritus. — Were the elevated portions of the earth, in a 
natural state, in reference to gravity, the soil formed by decom- 
position and disintegration, would remain in its place, and pro- 
tect the subjacent materials from farther changes. But the por- 
tions loosened, of whatever size, have a tendency to descend 
from their unnatural elevation ; and they are aided in their pro- 
gress by the action of rains, frost and snow. The accumulations 
which occur, flanking the acclivities of mountain-ranges, are of 
this description. The extent of their encroachment on the low 
land, and the peculiarities of their arrangement, have usually 
been effected by the agency of running water. The rivers, whe- 
ther raging in fury during rains and thaws, or gently descend- 
ing, during droughts, are still contributing to transport the dis- 
integrated materials of the mountains towards the plains, car- 
rying forward the larger masses to short distances ; but the mi- 
nute portions to their lowest level, to form islands and banks at 
the estuary. The strata of detritus thus formed, are necessari- 
ly irregular in their composition and structure, and contain the 
relics of the animals or vegetables of the river district, or accli- 
vity, in which they are situated. Detritus is subject to be co- 
vered with soil and sand-drifts ; and, in many cases, repetitions 
of these strata have taken place. 
