1 8 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



inclined surfaces may be clothed with rough grit and gravel, 

 consisting largely of insoluble quartz. Over the low grounds 

 clay is sometimes spread to some extent ; but from this the 

 soluble alkalies and alkaline earths may have been largely 

 removed. 



Basalt is a dark-coloured rock which, as a rule, is readily 

 disintegrated and decomposed. It contains a considerably 

 smaller percentage of silica than granite, and a much larger 

 proportion of basic ingredients. The essential constituents are 

 felspar and augite — usually accompanied by olivine and iron- 

 oxides, and not infrequently by apatite, but no quartz is present. 

 From the decomposition of these minerals various soluble and 

 insoluble compounds are produced. The former are represented 

 chiefly by carbonates of lime, potash, and magnesia, while the 

 latter appear as clay and fine sand, commingled with which are 

 iron-oxides. The resulting soil is a dark loam, which often 

 shows traces of phosphoric acid, derived from the decomposition 

 of the apatite. Basalt-rocks are fairly common in the lowlands 

 of Scotland, entering often largely into the formation of hill- 

 ranges, and frequently occurring as more or less isolated knolls 

 and straggling hillocks, or forming wider stretches of lumpy and 

 hummocky ground. 



Granite and basalt may be taken as representative of igneous 

 rocks in general — the former yielding, under favourable conditions, 

 clay-soils, while from the latter, soils of a loamy character tend 

 to be produced. There are many other igneous rocks, however, 

 a few approximating to granite in chemical composition ; others 

 being more nearly related to basalt. Rocks of this intermediate 

 character necessarily yield soils — some of which are clay-like, 

 while others are loamy. Generally they are more or less fertile. 



2. Schists. — The rocks included under this head are of very 

 variable nature, and are often termed " metamorphic," because 

 they are believed to have undergone certain great changes which 

 have obscured or even sometimes entirely concealed their 

 original character. The most typical kinds are markedly 

 crystalline; but others may show only slight indications of 

 crystalline texture. 



Gneiss is one of the more crystalline varieties, and resembles 

 granite in composition, consisting, like it, of quartz, felspar, and 

 one or more ferro-magnesian silicates. It therefore tends to 

 yield a clay-soil with the usual admixture of grit. According, 



