

284 STRUCTURAL AND FIELD GEOLOGY 
and the geologist, therefore, in drawing his boundary-lines, 
must make the necessary allowances. In the case represented 
in the diagram there would be no difficulty in ascertaining the 
boundary lines between the several beds. Walking up the 
slope the presence of rounded stones would indicate the 
presence of the conglomerate, so long as even one or two only 
appeared. Above the junction of beds c and 6 water-worn 
stones would no Jonger be met with, while fragments of sand- 
stone might continue to abound until the limits of the stratum 
6 were reached. The position of the boundary-lines to be 
drawn would thus be approximately indicated. 
Although the colours of soils are invariably due to the 
character of the rocks from which they have been derived, 
the observer must remember that the colour of unweathered 
rocks often differs greatly from that of their disintegrated 
débris. The brown and reddish colours of many soils are 
due to the presence of iron-oxides, but such soils are often 
derived from rocks which are neither brown nor red—these 
colours having resulted from the chemical alteration of the 
rocks. Many basic igneous rocks, for example, which may be 
dark blue or even black, yield yellowish and reddish-brown 
soils. Again, some kinds of blue and grey boulder-clay are 
overlaid with reddish-yellow soils. Many impure blue and 
grey limestones also tend to yield yellowish or brownish soils. 
Generally speaking, however, the colour of soils formed from 
the disintegration of derivative rocks does not, for obvious 
reasons, differ much from that of the rocks themselves. 
(b) Character of Vegetation.—The character of the vegeta- 
tion is often an index to the nature of the soil and underlying 
rocks which the observer cannot always afford to neglect. It 
is well known that certain plants prefer one kind of soil to 
another, so that botanists are able to map out a region into 
areas (not always, it is true, sharply defined), each of which 
is distinguished by the development of some particular 
assemblage of plants, or by the presence of certain plants 
and the absence of others. As the distribution of these 
plant-societies depends mostly on the chemical and physical 
conditions of the soil, it is necessarily suggestive to the 
geological observer. Soils poor in carbonate of lime show a 
different assemblage of plants from those which are rich in 
