
390 STRUCTURAL AND FIELD GEOLOGY 
percentages of clay,and may form loamy soils: of excellent 
quality. _From loams capable of. high cultivation, we pass 
on to clays, many of which are tenaccous, although few 
alluvial clays have the tenacity so charaereent of the tills 
and stoneless clays of glacial origin. Alluvial clays and 
muds often contain much organic matter, and are frequently 
rich in soluble mineral salts. As examples of alluvial for- 
mations may be mentioned the flats and terraces of our 
river valleys, the great Carse-lands of Middle Scotland, the 
raised beaches of our maritime tracts, and the many patches 
of level ground which indicate the sites of former lakes. 
Under the same head, also, we may include the fluvio-glacial 
heaps and sheets of gravel, sand, etc., shortly described in 
Chapter XX. Although these deposits are primarily of sub- 
elacial derivation, yet their materials must obviously have 
been more or less altered and disintegrated while they were 
being transported and distributed. Moreover, their highly 
permeable nature has allowed the free passage of rain, so that 
in time all such fluvio-glacial gravels and sands have acquired 
much the same character as the similar deposits formed by 
ordinary river-action.* From an agricultural point of view, 
therefore, they may be included under one and the same head. 
In fine, it will be understood that the chief distinction 
between “alluvial-formations” and those which have been 
described under the head of “glacial soils,” is simply this, 
that the former consist essentially of “weathered” rock- 
material, while the latter are composed almost exclusively of 
“unaltered” mineral matter—of crushed, pounded, and pul- 
verised rock, which had previously undergone little or no 
chemical alteration. The “alluvial formations,’ in a word, 
consist of disintegrated rock-material, and are true “subsoils.” 
Glacial clay-deposits, on the other hand, are not “subsoils,” 
but, properly speaking, unaltered “ bed-rock.” 
* It may be noted, however, that the stones of a fluvio-glacial gravel 
are usually fresher than.those of modern alluvial deposits. Their 
weathered crusts are thinner, and they are sounder internally. This 
is most notable in the case of basalt and similar rocks. When the 
stones of a modern gravel-bed have been derived directly from till, 
however, they are usually quite as sound as those occurring in a fluvia- 
glacial deposit, 

