TRAPPEAN SOILS. 241 



its color darker than either of the preceding. It is also warmer 

 and more favorable to cultivation. 



8. Hornhlcndc rock soil. Hornblende rock is composed 

 chiefly of hornblende and compact feldspar, with variable 

 portions of oxide of iron and of manganese, and the soil is 

 composed of similar materials. The color is generally of a 

 dark red-brown, of a fine texture, slightly adhesive when 

 pressed in the hand, but not clayey. This soil contains a 

 large quantity of oxide of iron, manganese and magnesia, the 

 latter substance supplies the place of clay ; the manganese, 

 from its dark color and imperfect conducting power, renders 

 the soil warm and highly fertile. 



9. Porphyry soil is derived from the compact feldspars, 

 which contain from 25 to 30 per cent, of alumina. The por- 

 phyry rock is among the hardest, but it yields rapidly to the 

 agents of disintegration, and forms a very valuable soil. 



VI. Trappean soils differ from the preceding by contain- 

 ing from 3 to 7 per cent, more of lime, magnesia and iron, and 

 20 per cent, less of silex. 



1. Greenstone soil is often associated with porphyry. It 

 is of a finer material, and more fertile. The character of 

 these soils is often distinct, of a brown color, containing large 

 quantities of iron. Basaltic soil is very similar to the above, 

 but it is composed of augite and feldspar. 



2. Trachyte soil. This is the soil from the ancient lava, 

 and is found around volcanoes. It contains a large quantity 

 of alkalies, which make it highly fertile. It is composed of 

 glassy feldspar, hornblende, mica, titaniferous iron, and some- 

 times augite. 



3. Lava soil. The more recent lava, when converted in- 

 to soil, is often very fertile. It contains so large quantities of 

 alkali, such as potash, soda, etc. that for some crops, it is the 

 best of all soils. The two minerals, feldspar and augite, con- 

 stitute nearly the entire mass of this soil. As these matters 



