

380 STRUCTURAL AND FIELD GEOLOGY 
rocks. Quartz-porphyries and rhyolites yield soils of much 
the same character—they are essentially clays with a larger 
or smaller percentage of sand (quartz) and not infrequently 
with a notable proportion of potash, magnesia, and lime. 
But, just as with granite, the character of the soil-cap is 
largely determined by the configuration of the ground and 
climatic conditions. In short, the soil-cap, according to 
circumstances, may be a fine sandy clay, or a mere rubble of 
sand, grit, and rock-fragments. 
Basalt.—As granite is the type of the acidic igneous rocks, 
so basalt may be taken as representative of the basic series, 
The essential constituents of this rock, it will be remembered, 
are felspar and augite (usually with olivine), and generally a 
considerable proportion of magnetite (often accompanied by 
ilmenite). The rock commonly weathers to some depth, 
becoming so disintegrated that it may be readily dug with a 
spade. The resulting soil is a dark-coloured loam—the more 
notable constituents of which are clay, fine sand, iron-oxides, 
and varying proportions of the carbonates of lime, potash, 
magnesia, together often with traces of phosphoric acid, 
derived doubtless from the decomposition of apatite—a 
common accessory mineral in basalt as in many other igneous 
rocks. Where the surface conditions are favourable, basalt 
always yields rich soils of this character. 
Diorite, Porphyrite, etc.—TVhat large class of igneous rocks, 
the silica percentage of which is less than that of the granites, 
quartz-porphyries, etc., but larger than that of the basalts and 
their associates, yield soils of an intermediate character, 
which would be classed rather as loams than clays, and are 
often highly fertile. The diorites and porphyrites are 
essentially compounds of soda-lime felspar with various 
ferromagnesian minerals, such as hornblende, augite, biotite, 
etc. The rocks do not, as a rule, weather so readily as 
basalt, but this is not always the case, for now and again 
their decomposed crusts and débris can hardly be distinguished 
‘from disintegrated and decomposed basalt. Generally, 
however, the soil derived from these rocks of intermediate 
composition contains a less percentage of iron-oxide than 
basalt-soil, and is usually more clay-like. The subsoils, as one 
might have expected, are rich in lime derived chiefly from the 

