

388 STRUCTURAL AND FIELD GEOLOGY 

boulder-clay soils, however, are upon the whole the best. 
These consist mainly of pulverised red sandstone, and form 
strong loams, rather than clays. Chalky boulder-clays, com- 
posed chiefly of pulverised chalk, yield clay-soils from which 
the calcareous constituents have sometimes been largely 
removed. 
The agricultural treatment of soils is a subject on which 
the geologist has no title to speak. He may, however, be 
allowed to point out the danger of deep-ploughing upon 
boulder-clay soils of all kinds. Undisturbed boulder-clay 
consists almost exclusively of unweathered materials—its 
mineral constituents are quite unaltered, and it is therefore 
in no sense of the term a subsoil. It plays the part, in fact, 
of unweathered “bed-rock.” Owing to its impervious 
character, the subsoil and soil formed upon it seldom exceed 
a foot or two in thickness. Now and again, as.in sandstone 
regions, it is somewhat more pervious, and covered, there- 
fore, with a thicker soil-cap. In some boulder-clay tracts, 
indeed, the arable soil considerably exceeds a foot. It will 
be found, however, that these thicker soils have not been 
derived exclusively from the boulder-clay upon which they 
lie, but have in large measure been washed down by rain 
from adjacent slopes. This is shown not only by the unusual 
thickness of the soil in question, but by the fact that it 
contains relatively few stones, while the much thinner soils 
of the neighbouring banks and mounds are crowded with 
stones, the tops of the banks being not infrequently covered 
with a thick sheet of course shingle and boulders. 
Stoneless Clays.—These deposits consist usually of very 
fine gutta-percha clays. They are generally well laminated, 
and are confined in these islands to maritime districts, where 
they seldom occur more than 130 feet above the sea-level. 
They are best developed in the lower reaches of the great 
valleys of Central Scotland, where they form a considerable 
proportion of the Carse-lands of the Tay, the Forth, and 
the Clyde. Clays of precisely the same character are met 
with in the Newcastle and Durham districts. The deposits 
have occasionally yielded shells of Arctic molluscs, and now 
and again an isolated stone or boulder appears: in a few 
places, indeed, small and large erratics are of quite common 

