

384 STRUCTURAL AND FIELD GEOLOGY — 
therefore, that they must become decomposed, and that their — 
soluble alkalies and alkaline earths are available for the 
support of plant life. Some sandstones contain so much 
argillaceous matter that their weathered materials form clay- 
like rather than loamy soils. Such is usually the case with 
the paleozoic greywackés, which are only much indurated 
argillaceous sandstones. The soils they yield are usually cold, 
retentive clays. Owing to the fact that these rocks occur, 
as a rule, in high-lying districts, their soils are seldom tilled. 
In low-lying districts, however, soils of the same origin, when 
they can be well-drained, are cultivated with success. Grey- 
wacké, it may be added, often contains much felspathic 
material, which, on decomposing, supplies alkalies and alka- 
line earths. | 
Argillaceous Rocks.—These naturally give clay-soils, but, 
owing to the variable character of the rocks, there are as 
many differences among clay-soils as among arenaceous soils. 
Some argillaceous strata contain so much sand, that the soil 
resulting from their disintegration might be classed among 
the loams or transition soils, being neither clays nor sands. 
Not a few clay rocks consist almost entirely of clay and 
quartz in the very finest state of division, all soluble ingredients 
being practically wanting. The soils formed upon these are, 
it need hardly be said, very infertile. Certain argillaceous 
rocks, on the other hand, may be largely charged with 
calcareous matter, and would be then termed marls, some 
of which yield excellent soils. It may be noted here, how- 
ever, that the term mar/, as used by some geologists, is 
misleading. Many of the so-called “marls” of the Old Red 
Sandstone, the Permian, and the Triassic systems contain 
no carbonate of lime, but are simply clays with a larger or 
smaller percentage of sand. As they occur interbedded with 
sandstones, the overlying soils usually assume the character 
of a “strong loam,” forming what is one of the most fertile 
soils met with in these islands. Good examples are furnished 
by the famous “red soils” of East Lothian, Wales, and 
Cornwall, all of which overlie rocks of Old Red Sandstone 
age, and the somewhat similar soils (“red ground”) yielded 
by the Triassic Keuper Marl of Cheshire and the Mid- 
lands. Clay-rocks in general, however, tend to give heavy 
