( 93 ) 



and thus dissolves and returns to the soil actually more crude nutriment for the plants than the 

 icild vegetation requires from it ; hence the increasing fertility of fallow lands. The heaviest raiu 

 cannot carry any considerable quantity of these down into the subsoil, except when in a 

 certain excess (which in nature probably never occurs). Diluted brown-coloured sewage 

 filtered through garden soil parts with nearly all its ammonia and potash and entirely loses its 

 phosphoric acid ; it flows off in the form of a colourless and odourless water. Soil purifies 

 water in the same way as charcoal. It is quite diiferent in the case of detritus of rocks, however 

 fine this may be. Here the rain water charged with carbonic and nitric acid is not only per- 

 mitted to flow off unchanged, but it will even dissolve minute quantities of the rock particles 

 and carry them down in solution. If we find a strictly calcareous plant growing on a pure 

 silicious soil, we have to remember such relations ; for although chemical analysis may indicate 

 only vestiges of carbonate of lime in this soil, the supply of the same substance by rain and 

 mist, may be as sufficient, as is the supply of it in the ocean for the construction of those 

 colossal coral-reefs that encircle so many islands. Funaria, a little moss, is found frequently 

 growing on the p^Aster of walls, but still more so on recently burnt places, where it occurs so 

 regularly and copiously that it is called by the French la charbonniire. Several other mosses and 

 a tew fungi, like Xylaria, grow actually on the pure charcoal of burnt trees, and equally vigor- 

 ously on brick masonry. Many so-called ammoniacal plants (chiefly weeds) grow abundantly 

 in places rich in ammonia, around human habitations, and grow vigorously and luxuriantly 

 there in the shade ; but the same plants are seen still more copiously on ruined pagodas, 

 houses, etc., and again along the sides of brick roads, here often reduced in size or of 

 meagre growth, but healthy. The peepul tree grows almost on every soil, and is seen as 

 freely, growing from the ruins of brick buildings and from the plaster of walls, as it is 

 on trees where a little humus-soil originated by the decomposition of the bark, etc., is quite 

 sufficient for its young growth. When we meet with such variation, we might get confused 

 and be tempted to reject chemical as well as physical influence, but if we carefully consider the 

 quality which all these supporting media have in common, viz., that of absorbing ammonia and 

 nitric acid from the air and rain, we shall understand the real cause. 



2. Physical nature of siibdratum. The surface soil is not of such importance for trees 

 as it is for herbaceous plants and, generally, for agricultural produce. It is the subsoil or 

 rather substratum that is of essential importance. Nor can one with any certainty conclude 

 from a certain surface soil what the substratum is, more especially, where alluvial or 

 diluvial beds overlie strata of older rocks. For instance, a deep alluvial clay may chemically 

 and physically shew in two localities quite the same character, but the tree vegetation on 

 these localities may differ very greatly. By boring, however, it may soon be found that 

 whereas one locality has a thin layer of retentive plastic clay, the other has fine loose quartz- 

 sand beneath its subsoil ; the former possibly calling swamp forests into existence, while the 

 latter may bear savannah forests. 



For present purposes we may class the various rocks* in the foUowingt order. 



(1.) Igneous rocks, such as some greenstones, trachytes, granites, etc. are closely allied 

 in their mineral composition to some metamorphic crystalline schists, as gneiss, mica schist, etc. 

 Although scientifically inadmissible, I designate them indiscriminately as metamorphic for 

 the sake of brevity. The influence which these various rocks exercise has not yet been properly 

 studied within the tropics. The vegetation on such is regulated by the prevalence of certain 

 minerals which compose the rock and by the greater or lesser compactness and permeability. 



The mechanical structure of such rocks, whether they are composed of several minerals 

 in coarse grains or even crystals, like many granites, etc., or whether composed of microorysta- 

 line or of only a very few minerals shewing at the same time a more uniform and compact 

 structure, exercise a great influence upon the growth of trees. Exposure and the degree of 

 humidity of the atmosphere along with the degree of light are the most powerful regulators 

 ef vegetation in districts consisting only of such rocks. They are botanically positive rocks, 

 inasmuch as they produce the most varied vegetation with reference to species, although 

 the vegetation itself may sometimes be poor. 



2. Sedimentary rocks, often differ a good deal in their relationship to the vegetation 

 from the former series of rocks, and are, to a certain degree, often much poorer in vegetative 

 forms. Amongst the sedimentary rocks are : 



* Those desirous of making themselves more generally acquainted with rooks, will find the undermentioned 

 book very useful, but, of course, rocks cannot be studied from a book alone : 



B. V. Cotta, Rocks classified and described, Lond., 1866. 



t I Have given here the names of only such rocks and formations which I have found to affect 

 tree-vegetation in India more or less. From the sequel it will become clear, that one and the same rock may 

 in different climates bear a vegetation which on comparison by no means agrees with what we expected. 

 Hence the necessity of studying the vegetation on the same rock in various countries and of making oneself 

 acquainted with the pecuharities of one and the same substratum under different climatological conditions. An 

 arranf ement of the various rocks into groups that have the same or a similar effect upon vegetation is, at least for 

 the present, simply impossible. 



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