On the Food of Plants. 
501 
of limestone of various kinds, and in some particular localities 
from coral rocl<s, which we cannot regard otherwise than as 
incipient limestone formations, inasmuch as many of our older 
beds of that substance have certainly had such an origin, as their 
orofanic remains testify. 
Limestones differ very much indeed in their power of resisting 
disintegration : some, such as the hard blue mountain limestone 
beneath the coal-beds, resist more effectually than the hardest 
granite ; and districts formed of this material are distinguishable, 
even at a distance, by the scantiness and poverty of their vegetable 
covering, while, on the other hand, the softer beds, such as the 
oolites and the chalk, give way much more readily and furnish 
tolerably fertile land. The mode in which the naked coral reefs 
of the Pacific become in time clothed with vegetation, has been a 
favourite subject with voyagers in those parts ; they describe to us 
how the seeds of plants which are unaffected bv salt-water, such 
as cocoa-nuts, get carried by the waves from place to place until 
they are ultimately thrown high and dry into some crevice on the 
top of the reef, already in a slightly disintegrating condition ; how 
other seeds are brought by birds, and so forth, until at length the 
rock becomes covered with verdure : a lodgment being as it were 
thus made, the vegetable invaders acquire stronger and stronger 
hold, until at length a soil has been formed capable of supporting 
all the ordinary productions of the climate. 
3. Sand is produced by natural operations in two ways. — 
\yhen disintegrated felspathic rock is exposed to the natural 
elutriation of a stream of water, the materials of which it is com- 
posed are carried onwards by the stream to very different distances: 
a slight diminution in the current causes the deposition of the 
c[uartz grains. Sic, while the finely divided clay is conveyed 
much further, and only settles down when the water is tranquil : 
now sand of this description is recognised by the angular character 
of its grains — they are irregular quartz crystals in point of fact. 
The mechanical attrition of water is, however, by far the most 
abundant source of this substance. All ordinary sand, when 
examined by a magnifying-glass, sufficiently points out by the 
smooth rounded aspect of its grains its origin in the abrasion 
resulting from the rubbing of masses of stone against each other 
by the action of water in motion. A glance at what is constantly 
going on at every sea beach and in every water-course will set 
this matter at rest. The nature of the sand will, of course, de- 
pend upon that of the rock from which it is derived ; and thus we 
may have it siliceous, as in all ordinary cases ; calcareous, as on 
the beach of a coral island ; micaceous, &c. 
A large proportion of the earth's surface is covered with various 
irregular beds of sand, gravel, and clayey matters, which have 
