38 A CHAPTER IN THE PHYSICAL GEOGRAPHY OF THE PAST. 
sophical naturalist for studying geographical distribution, 
and for working out all those great influences of climate, 
soil, and general external conditions, upon the varietal changes 
which occasionally become stereotyped as new species. In 
order to facilitate this highest aim of the naturalist, our local 
societies ought, I think, to make their lists more gwantitative 
in character, if that is possible, and to note more than they 
do at present the interdependence of animal and vegetable life, 
and the relations of the plants to the geological nature of the 
soil upon which they are found. The field geologist often gets 
valuable hints as to the character of the rocks hidden under 
a thick mantle of vegetation, by observing the nature of the 
plants growing upon them. Plenty of instances of this kind 
must occur to any geologist who has occupied himself with the 
minute survey of a district. As a good example, I may mention 
that Professor A. H. Green, during his survey of the Carboni- 
ferous Rocks of North Derbyshire, found that he was often able 
to define the boundary of the Carboniferous Shales and Sand- 
stones by the fact that rushes are found on the shales, and that 
heath and furze grow more plentifully on the sandstones ; and he 
also notes that a crowded belt of the little Viola /utea is often seen 
along the outcrop of a sandstone bed, whilst not a single plant 
will be found on the shales that come out on either side. 
The aid which botany is able to afford to geology is manifestly 
reciprocal, and if botanists would only take care to note, amongst 
other things, the kind of sub-soil upon which any particular plant 
grows, they would, I think, render their lists more valuable, and 
attach to them a far wider interest than they commonly possess. 
These are mere suggestions which I venture to make for your 
consideration, but I will not enlarge upon them this evening, 
as I wish to occupy your time in trying to deduce from a mass of 
geological facts, which to many of you must seem dry and 
uninteresting, certain generalizations which | trust will prove of 
interest even to those who have no special knowledge of geology. 
The student of geology, whilst occupied in observing the thick- 
ness of the strata, their physical nature, order of superposition, 
