396 MR. T. W. WOODHEAD ON THE 
rises to 1200 feet. The rocks consist of fine-grained sandstones 
alternating with extensive deposits of shales and clays. The 
soils are often deep, frequently covered with much humus, and 
retain much water.  Climatie conditions are medium, and 
although the rainfall is much lower (33 inches) than in Zone 2, 
owing to the nature of the soil water is more constant and the 
vegetation is mesophytie, except on soils over sandstone and in 
the higher parts, where xerophytes extend from the Gritstone 
Plateau. 
In a district like the one under consideration, where Glacial 
deposits are absent, and soils owe their origin largely to the 
denudation of the underlying rocks, a solid-geology map is of 
ereat value when making a primary analysis. It has here been 
shown that a comparison between such a geological map and a 
vegetation map is very instructive and suggestive ; that a change 
in geological formations and soil-conditions is accompanied by a 
change in plant-associations. 
It has been shown that carefully selected small areas studied 
in detail, and the results shown on lines such as the present 
Wood-maps, help considerably to bring out the main factors 
affecting plant-distribution ; and it is believed that such maps 
could be usefully introduced in connection with larger surveys 
to supplement the features illustrated in small-scale maps. The 
results indicate that the study of small associations will materially 
aid the study of plant-geography, and bring to light many inter- 
esting points in the life-histories of the species. 
The present study indicates that, in this district, the physical 
properties of the soil and accompanying conditions play a more 
important part in determining the character of plant-associations 
and the distribution of species than the chemical composition of 
the soil. 
The dominant elements of an association tend to form a 
biological unit, and in the case of the Meso-pteridetum (Scilla, 
Holcus, and Pteris) dealt with we have a complementary associa- 
tion, the subaerial parts being in, or tending to occupy, definite 
and different layers, i. e. edaphically complementary, and the 
aerial parts are seasonally complimentary. 
On the other hand, we have in the Xero-pteridetum an associa- 
tion where the dominant species (Vaccinium, Calluna, Pteris, and 
