GEOLOGICAL FORMATIONS. 107 
«The mechanical condition’ and physical peculiarities of 
the soil will also be found to modify the effects of the 
influential circumstance already referred to, and contribute 
to:chain particular plants to particular soils, or to facilitate 
their dispersion. 
« €There are plants which will only settle on unbroken 
rocks, and which, when the other conditions coincide, spring 
from these rocks to our walls, like the wall rue spleenwort 
(Asplenium Ruta muraria), a little fern, the name of 
which. denotes its station. Others occur only where 
weathering has broken up the solid rock into small frag- 
ments—drift plants, which, clinging to mankind, select 
rubbish heaps, which most resemble their natural station : 
the great nettle and the henbane serve as examples. 
: ‘Lastly, other plants grow only where the ‘rocks have 
been reduced to fine powder in sand, or in the fine- 
grained clay produced by chemical decomposition. The 
so-called. German sarsaparilla, the sea weed (Ammophila 
arenaria), is an example of the first condition, but there is 
no definite condition corresponding to it in the vicinity of 
human habitations. 
- *Clay, on the other hand, stands beside the black sub- 
stance, humus, resulting from the decomposition of organic 
matter. Both rich in soluble salts important to vegeta- 
tion, both distinguished in their property of absorbing. 
from the atmosphere, and their conveying to the root of 
plants, gases and aqueous vapour, they cause, single or in 
combination, the most luxuriant vegetation. And we thus 
obtain three stages in reference to the qualities of the soil 
~——pure earths, wholly devoid of vegetation; mixed earths, 
without clay or humus, with an arid but characteristic. 
vegetation ; and lastly, soil rich in clay and humus, with 
the greatest. abundance and variety of plants. 
‘Even in the north the eye of the uninstructed observer 
is struck by the greater richness and stronger development 
of the vegetable kingdom, from the argillaceous, basaltic, 
and porphyritic soils; and under the tropical sun, simple 
quartz sand is a desert, if water, and therein foreign 
