TRANSACTIONS OF THE SECTIONS. 43 



posited from aqueous solution, though occasional impregnation of previously existing 

 rocks by percolation was by no means unlikely. 



On the Properties and Composition of the Cocoa Leaves. 

 By Professor Johnston, M.A., F.R.S. L8fE. 

 After describing the remarkable physiological properties of the leaves of this plant, 

 the author explained that they yield to aether a peculiar volatile resinous substance 

 possessed of a powerful odour, in which the peculiar virtues of the leaf are supposed 

 to reside. The plant is as jet to be obtained in too small quantity in this country to 

 admit of a complete chemical examination of the substances which the leaves contain. 



On the Causes, Physical and Chemical, of Diversities of Soils. 

 By Professor Johnston, M.A., F.R.S. L. 8f E. 



In this paper, the author, assuming as a general rule that the materials of which 

 soils are composed are derived from the rocks on which they rest, and that therefore 

 the agricultural is very materially dependent upon the geological character of a 

 country, showed how physical and chemical influences subsequently interfered almost 

 everywhere materially to modify the agricultural indications of geology. 



I. Among physical influences, he showed — 



1. How the flatness of a country and the absence of outfalls causes the rain-water 

 to stagnate, covers it with bogs, and obliterates the agricultiu-al influence of the rocks 

 beneath. 



2. How high and sloping lands yield their finer particles to the rains which fall 

 upon them, to be borne down to lower levels. Thus the granites yield their felspar 

 and the red sand their fine marls, and thus from the debris of the same rock, often 

 extending over large areas, regions of very different soils are established. 



3. How along the line of ancient or existing water-courses, a ribbon of varying 

 breadth is found in every country, upon which the soils consist of these fine matters 

 separated and sorted by the action of water, and possessing agricultural characters 

 more or less different from those which naturally belong to the geological formations 

 on which they rest. And these differences become the more marked along the courses 

 of great rivers, or of such as descend from great distances, and flow through various 

 geological formations, of which they wash out, bear away, and intermingle ihe debris. 



4. How along the shores of the sea, successive elevations of the sea establish upon 

 the same geological formation belts of sand, very unlike in agricultural value. 



These remarks were illustrated by an agricultural map of New Brunswick ; and the 

 conclusion the author endeavoured to establish was, that the physical geography, the 

 hydrography, and periods of elevation of a country were scarcely less important than 

 its geological structure in determining the agricultural value of its surface. 



II. Among important chemical influences, the author mentioned as of much weight — 



1. The production of acid matters in the soil wherever vegetation existed. Such 

 acid matters are constantly produced wherever vegetable substances undergo decay 

 in the surface soil, and sometimes in such quantity as to render the soil sour to test 

 paper. These acids are washed downwards by the rains which sink into the soil. As 

 they descend, they dissolve out of the soil such earthy substances — lime, alumina, oxide 

 of iron, &c. — as they are capable of taking up, and these they bear away with them 

 in a fluid form wherever they flow. Thus they gradually establish differences, both 

 chemical and physical, between the upper and under portions of the drift or rocky 

 debris from which the soils are formed, and at last render the uppermost layers in 

 which the plants grow, totally different in its agricultural character from that which 

 belongs to the original unaltered materials themselves. Hence the thin non-calcareous 

 soils which cover the chalk and other limestone format ions — the constantly recurring 

 necessity for the re-addition of lime to cultivated land — the benefits from bringing up 

 new soil from beneath, and of many other agricultural practices. 



2. The firing of the forests in new countries, where hot summers scorch vegetation 

 and raging fires spread their devastations sometimes over thousands of square miles. 

 Such fires are almost invariably attended by sti'ong winds, which bear away the ashes 

 of the burning wood to immense distances. Thus in a single day all that the trees 

 have been extracting from the soil during a whole half-century is swept away ; even 

 the surface soil itseu is sometimes scorched and swept bare. After a time a new 



