

GEOLOGICAL SURVEYING 285 
that substance. There are certain species (¢g. common 
bracken, common heather, sorrel, fox-glove, etc.) which avoid 
calcareous soils; while, on the other hand, not a few species 
(e.g. wild cherry, beech, dogwood, and many flowering plants) 
are particularly partial to such soils. Porous sandy soils, 
tenacious clay, loose loams, saline soils, etc., are each char- 
acterised by the presence of distinctive plant-groups. In the 
absence of rock-exposures, therefore, the plant-associations 
referred to may often be helpful to the field-geologist, and 
enable him to draw his boundary-lines with more or less 
eenlidence. He must bear in mind, however, that the 
boundary-lines suggested by the varying character of the 
vegetation will not often coincide even approximately with 
the junction-line he is in search of. Soils, we have seen, 
tend to travel down slopes, however gentle these may be, and 
in this way a soil rich in lime may eventually come to overlie 
a quartzose sandstone which might contain hardly a trace of 
lime; just as, on the other hand, a barren, infertile sand may 
in time invade and cover rocks, which, if left exposed to the 
weather, would naturally have yielded a highly fertile soil. 
Nevertheless, the observer who has a sufficient knowledge 
of botany will not infrequently have occasion to turn this 
knowledge to good account. Having ascertained the 
character of the flora which he finds growing upon soils in 
places where their derivation from the underlying rocks can 
be seen, as it were, taking place, the appearance of a like 
plant-assemblage elsewhere will lead him to suspect the 
presence of the same rocks below, although none of these 
may be actually visible at the surface. 
(c) Form of Surface-—The shape or configuration of the 
ground is frequently of great service in showing where a 
boundary-line should be drawn. As will be set forth more 
fully in the sequel, the forms. assumed by a land-surface are 
determined in chief measure by the nature of the underlying 
rocks and their geological structure. Rocks differ greatly as 
regards durability—some being much more readily reduced 
than others by the superficial agents of change. Hence, in 
regions which have been long exposed to denudation, the less 
readily disintegrated rocks tend to project, while the more 
yielding kinds are correspondingly worn-down or levelled. 


