PHYSICAL ASPECTS OF ADEN. 
31 
weathering, which may be either disintegration or decomposition. 
Though these two processes usually go hand in hand, it may generally be 
observed that one is more prevalent than the other. An examination of 
the surface of the hard volcanic rocks of Aden and Little Aden will at 
once reveal the fact, that decomposition is not very active, whilst the 
watercourses with their boulders and gravel show to evidence that 
disintegration, i.e., the breaking up of the rock into fragments of various 
size, is continually at work. It is only after a long period that the 
minor fragments exhibit distinct signs of decomposition and begin to 
form soil. Besides the natural hardness of the Aden-rock and its power 
of resistance against climatic influences, decomposition is greatly retarded 
by the almost entire absence of Lichens. On the highest peaks of the 
Alps and other lofty mountains, in the temperate as well as in the 
tropical regions, we find many species of crustaceous Lichens, which con- 
tribute materially to the weathering of the rocks and to the formation of 
a vegetable soil. But at Aden very few species are represented forming 
here and there some small, almost invisible patches. 
As the water is found in the form of thin films surrounding the soil 
particles, the amount necessarily increases with the increase of the water- 
holding surface. The latter is increased as the particles become finer 
and more numerous. At the same time the irregular capillary spaces 
between the particles grow smaller and, in consequence of it, the upward 
or capillary movement of the water is increased, whilst the downward 
movement or percolation is retarded. Judging from the geological and 
mineralogical characters of Aden and from the way in which, under 
similar circumstances, the formation of soil generally takes place, we 
must conclude, even if we did not know it by experience, that the soil is 
coarse-grained and highly porous. But porosity decreases the water 
content of the upper layers, the water not being absorbed, but passing 
through the upper layers in order to occupy the lower layers or to flow 
off as local circumstances may demand it. This is the reason why so- 
called dry habitats, such as prairies, gravel-slides, sand-hills, etc., have 
a low water-content, varying from 3 to 15 per cent. There is no doubt 
that the same obtains at Aden, not only with regard to the soils formed 
from the volcanic rocks but, a fortiori, regarding the sand fields of the 
isthmus and the British territory in general. 
As to the chemical composition of the soil of Aden there is not much 
to be said. The quantity and quality of soluble substances in all ordi- 
nary habitats are so nearly alike that differences in chemical composition 
are of little importance. The soil in the valleys, ravines, and water- 
courses of Aden and Little Aden, and wherever else there is a deposit of 
