( 9* ) 



(1.) Calcareous rocks, such as Hmestonee, dolomites, calcareoas sandstones, etc. 



(2.) Tufa and laterite formations, if ferruginous. 



(3.) Silicious rocks, such as silici'ous sandstones, many conglomerates, etc. 



(4.) Argillaceous rocks, such as slates, argillaceous shales, eta 



3, Detritus and soils. The principal varieties may be : 



(1.) Alluvial clays (silicious). 



(2.) Loams (more or less calcareous). 



(3.) Sands. 



(4.) Shingles, pebble-beds, etc., consisting of smaller or larger rolled stones. 



(5.) Peat, bog and turf.* 



(6.) Saline strata, whether alluvial or rocky. These have such a peculiar influence 

 upon vegetation, that they must be treated separately in all questions of soil. 



It is natural that formations consisting of a single mineral, like limestone, quartz, etc, 

 or if of sedimentary and uniform character, like sand with a chemically poor cement, will 

 have reduced the vegetation growing on them to a minimum, altliough this minimum may 

 conspicuously differ in quality. The limestone may have comparatively a greater number 

 of peculiar forms than are found even on metamorphio rocks, and may thus be positive 

 in this respect, but the grand total of species growing on it may be negative, while, on the 

 other hand, the permeable silicious sandstone of the Pegu Yomah has neither (or only a very 

 few) peculiar forms, nor has it a larger grand total of species, and is, therefore, like the deep 

 alluvium, decidedly negative in all its productions, when compared, for instance, with the 

 Martaban hills (metamorphic). 



Bocks that are mechstnically very similar such as conglomerates, breccia, and coarse sand- 

 stones, may resemble one another in the vegetation growing on them, while fine grained silicious 

 sandstones possess a flora similar to that found on alluvium, only richer in species and the 

 plants of better growth. Here the amount of permeability and the chemical combination of the 

 mineral constituents are the causal factors. 



The permeability of rocks, so often alluded to, is the chief cause of the formation of 

 surface soil. I have (although scientifically inadmissible) connected porosity, absorption, and 

 hygroscopicity, all under the general term permeable. Pernreability, as understood here, is a 

 combination of porosity or hygroscopicity with chemical solubility. Absence of the one or the 

 other of these qualities must result in greater or less sterility. Wherever I speak of 

 permeable, I mean physically permeable or hygroscopic rocks. Practically this genera/- 

 lization of permeability is useful, inasmuch as by such a process it becomes, for in- 

 stance, explainable, why the growth of a tree on poor fine quartz sand, (mechanically permeable 

 but physically impermeable) and that on laterite and other impermeable rocks should be 

 so similar or occasionally identical. The difiiculty in keeping the above mentioned scientific 

 terms apart, rests in the similarity of the qualities to one another, a similarity which however 

 exists, more in degree than in kind. 



Thurman's scale is constructed on the principle that the products of decomposition are 

 either divided indefinitely or only to a certain degree. The former he calls pelogeneous rocks, 

 because they form earthy, marly or clayey soils, while the latter are distinguished as psam- 

 mogeneous roeks, forming gravelly soils, such as quartz and other detritus forming rocks. If 

 both these conditions are represented in the product of decomposition, the rocks are called 

 pelopsammogeueous. The following is a conspectus of his scheme : 



A. Pelogeneous rocks : 



1. Perpelic, like Oxford marl, Keuper-thon, pure loam, pure Kaolin, etc. 



2. Hemipelic : calcareous marl. 



3. Oligopelic : Jura-calc, basalt, porphyry. 



B. Psammogeneoiis rocks : 



1. Perpsammio : quartz sand, sandy dolomites. 



2. Hemipsammic : molasse, grauwacke, granular limestone. 



3. Oligopsammic : certain granites, grauwacke, flysch, dolomite. 



C. Pelopsammogeneous rocks : 



Sandy loams, hemipelic porphjrry rich in quartz. Kaolin, granites, etc. 

 According to the amount of detritus which these rocks add to the soil, Thurman distin^ 

 guishes : 



1. Eugeogeneous rocks : perpelic, perpsammio, pelopsammio, hemipelic and hemipsammic, 



2. Dysgeogeneous rocks : oligopelic and oligopsammic. 



This docs not occur in India, at least not in the lower regions, and even those in the alpine regions differ 

 a good deal from tlie European. In tlieir stead humus-layers (often up to ^ foot thick and sometimes thicker) 

 appeu on trees, which nourish many epiphytic shrubs and herbs in damper climes aud at crtaiu elevations. 



