66 Arboriculture. 
recognized, are, that it does not effervesce in acids, it is harsh M'hen 
rubbed between the finger and thumb, and it cuts glass if rubbed 
against it. 
Alumina, (so called as constituting the base of alum,) occurs gene- 
rally in the form of stiff, retentive clay ; without a certain proportion of 
sand, it is scarcely permeable to water, and consequently, an unfit 
medium for vegetables ; but it does not occur in a state of absolute 
purity and minute division, and although it frequently requires an addi- 
tional portion of sand, to render it a proper medium for vegetables, I 
am not aware of its being in any instance, absolutely barren. The 
Agricultural character given of this clay, in Conybeare and Phillips's 
invaluable work on the Geology of England and Wales, is, that " It 
chokes the plough, and rolls before it, in a broken and muddy state ; 
after rain, it is not slippery, but adheres to the shoes ; after drought, 
it presents cracks nearly a yard in depth, and several inches in breadth. 
According to Townsend, it is sometimes called wood-gi'owers' land, 
because, although it is productive of the finest elm, oak, and ash tim- 
bers, it requires chalking before it can produce good corn; yet on 
Epping Forest, Windsor Forest, and much of the New Forest, the 
oaks are finest vv'here the clay is mixed with sand." It does not effer- 
vesce in acids, and when in a state of minute division, is unctuous and 
impalpable to the touch. It is known by the terms, argillaceous, clay, 
stiff retentive clay, and till. 
Calcareous earth results from limestone or chalk; — in tlie former 
case, the soil is always mixed with other ingredients, and is naturally 
suited to Agricultural purposes; hence the astonishing fertility' of many 
of the Irish counties. In the latter, it is occasionally very indifferent 
even in England, but on the continent, "according Cuvier and Brong- 
niart, sterility is one of its most decided characters, and Champagne 
is mentioned, as being, in some cases, absolutely uninhabiteble."'* It 
is easily distinguished fi"om the last, by its efferveschig in acids. Be- 
sides these, there are six earths enumerated by chemists, only one of 
v/hich, (J^Iagnesian) is foimd in sufficient quantity to modify in any 
considerable degree, the general nature of tlie soil, and this only in a 
few circumscribed districts. 
It therefore appears, that pure silica, alumina, or lime, are not ca- 
pable of supporting healthful vegetation ; and, as will appear, from a 
iew examples which I shall now quote, a mixtiu-e of these and of other 
substances, is necessary for that purpose. Bergman, an eminent 
French chemist, is of opinion, that the most fertile soils will generally 
be found to consist, as nearly as possible of 4 parts of clay, 3 of 
sand, 2 of calcareous earth, and 1 of magnesia. 
* Conybeare and Phillips's Geology. 
