
SOILS AND SUBSOILS | 389 
occurrence, but that is exceptional. The clays are obviously 
of marine origin, deposited at a time when Arctic climatic 
conditions obtained. When carefully examined, they prove 
to consist for the most part of minute particles of rock and 
mineral, which, as a rule, are as fresh and unaltered as the 
similar fine ingredients of boulder-clay. When thoroughly 
washed and sifted, the clay yields an exceedingly fine rock 
meal or flour, of which the most abundant constituent is 
quartz. ‘True clay or kaolin is of subordinate importance, 
but appears to be rather more abundant than in most true 
boulder-clays. These stoneless clays, therefore, would appear 
to be of the same origin as the former—they are the result 
of glacial grinding, and, unlike ordinary alluvial clays, are 
not the product of the mechanical and chemical process of 
weathering. They represent the fine mud, etc., swept into 
our estuaries by turbid rivers escaping from large glaciers, 
and too short a time elapsed before they settled down, to 
allow of much chemical alteration. It need hardly be said 
that the soils met with upon such clays are peculiarly 
tenaceous, except where, as sometimes happens; thin layers 
and bands of sand are intercalated in the upper part of 
the deposits. Deep-ploughing upon these clays is obviously 
not less to be avoided than in the case of true till. 
Alluvial Soils.—Under this head we include all superficial 
deposits consisting of disintegrated and weathered rock- 
materials, which have been transported and spread out by 
water. Some of these formations have been accumulated 
in fresh water, others have been laid down in estuaries and 
upon the sea floor. They and their soil-caps naturally vary 
much in character. The coarser deposits consist of water- 
worn shingle and gravel, and are generally barren, owing to 
the rapidity with which rain is absorbed. Any soil formed 
at the surface tends in this way to disappear. Now and 
again, however, when the interstices between the stones 
are well filled with grit and sand, a light porous soil is 
formed. Between such coarse accumulations and the finest 
deposits of mud and silt, we meet with all gradations. Of 
the sands, quartz is the dominant ingredient, and a pure 
quartz-sand, it need hardly be said, cannot furnish a good 
soil. Many sands, however, contain larger and smaller 

