soil: its origin and nature. 21 



sandstones and shales occur in rapid succession, the overlying 

 soil is usually of a loamy character. 



Argillaceous Rocks. — The rocks of this class are as variable in 

 character as those just described. Naturally the soils given by 

 them are clay-like. Many of the rocks in question consist 

 almost exclusively of silicate of alumina and quartz in the finest 

 state of division — all soluble ingredients being practically wanting. 

 Such rocks, it need hardly be said, crumble down into cold, wet, 

 retentive, infertile clays. Some clay-rocks, on the other hand, 

 contain much sand, so that the soil resulting from their dis- 

 integration is of a transition or loamy character. Again, certain 

 clay-rocks are largely charged with calcareous matter, and would 

 be then termed marls — some of which yield excellent soils. 



While argillaceous rocks sometimes occupy wide regions — the 

 prevalent soils of which are consequently of a heavy, clay-like 

 nature, yet more frequently the rocks in question are interbedded 

 with sandstones. When such is the case, the resulting soils, as 

 we have seen, are often strong loams, forming some of the most 

 fertile land met with in this country. Good examples are 

 furnished by the famous " red soils " of East Lothian, Wales, and 

 Cornwall, all of which overlie Old Red Sandstone strata. 

 Somewhat similar soils are yielded by the " Keuper Marl " of 

 Cheshire and the Midlands of England. 



Calcareous Rocks. — Limestones invariably contain insoluble 

 impurities, such as sand and clay — they do not consist exclusively 

 of calcium carbonate. Thus the soils derived from limestone are 

 either loams or clays. They are usually reddish, yellowish, or 

 brown in colour, and vary in character from stiff retentive clays 

 to calcareous loams. Good examples of these soils are seen in 

 the chalk districts of England. Limestones all the world over 

 yield somewhat similar coloured soils — the colour being due to the 

 presence of iron-oxides. As limestones are usually traversed by 

 numerous joints or fissures which have been widened by the action 

 of acidulated water, much of the red earth {i.e., the insoluble 

 portion of the rock) is washed by rain, or, as the case may be, by 

 melting snow, into these open fissures. Limestone regions, 

 therefore, especially when relatively high, are apt to show a bare 

 rocky surface, the hollows of which may be sparingly sprinkled 

 with a thin clay-like or loamy soil. 



\Note. — This article will be concluded in our next issue.] 



