134 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



In the first of these articles (see Vol. XX. p. 178) reference was 

 made to the fact that disintegrated rock-material is constantly 

 travelling, slowly or more rapidly as the case may be, from 

 higher to lower levels. This is, of course, most perceptible on 

 hill-slopes, and in places where the ground is periodically 

 ploughed, and the soil exposed to the mechanical action of the 

 superficial agents. The soil derived from some particular kind 

 of bed-rock or superficial formation must thus come to invade 

 and overlie subsoils and bed-rocks of a different character from 

 that which gave it birth. When we have recourse, therefore, to 

 a geological map as a guide to the nature of the bed-rocks and 

 superficial formations, it must be borne in mind that the map 

 does not profess to show the precise limits of the soils, but the 

 boundaries of the more important groups of bed-rocks, and over- 

 lying glacial and alluvial accumulations. For example, the map 

 may show that a certain hilly tract is occupied by some igneous 

 rock, say basalt. Over that tract, therefore, we may expect to 

 encounter a dark loamy soil, but this soil will naturally extend 

 down the hill-slopes, and thus cover a wider area than the rock 

 from which it originated. Again, it may be noted that the 

 colours upon geological maps refer, as a rule, only to the so- 

 called " solid rocks," and these may or may not appear at the 

 surface of the ground where they are represented by the map 

 to occur. Wide regions which may be shown on the map as 

 occupied by limestone, or by sandstones and shales, or other 

 kinds of bed-rock, may, in fact, be deeply buried under superficial 

 accumulations, and thus may have had no influence whatsoever 

 on the nature of the overlying subsoils and soils. The Govern- 

 ment Geological Survey, however, is now issuing maps which 

 indicate the actual areas where bed-rocks are exposed at the 

 surface, and the tracts over which superficial formations are 

 distributed. Agriculturists and others interested are thus 

 supplied with reliable data for forming some opinion as to the 

 general character of the soils that are likely to be met with in 

 the areas represented on such maps. 



