SILICATES OF ALUMINA. 20i9 



They must, therefore, be present in greater or less quantity in soils 

 which are directly formed from the decomposition of such rocks. Like 

 the silicates of lime, however — though more slowly than these — they 

 will undergo gradual decomposition by the action of the carbonic acid 

 of the atmosphere, and of the acids produced in the soil by vegetation 

 and by the decay of organic matter. The magnesia, like the lime, will 

 thus be gradually brought down, in a state of solution (p. 200), from the 

 higher grounds, or washed out of the soil, till at length it may wholly 

 disappear from any given spot.* 



6°. Silicates of Alumina. — Silica combines with alumina also in vari- 

 ous proportions, forming silicates, which exist abundantly in nature in 

 iJie crystalline rocks, and may also, like the other silicates, be formed 

 by art. Felspar, mica, hornblende, and the augites, which abound in 

 the trap-rocks, all contain much alumina in combination with silica, and 

 we shall probably not be very far from the truth in assuming that up- 

 wards of one-half by weight of the trap-rocks in general — as well as of 

 the hornblendes, micas, and felspars, of which so large a part of the 

 granitic rocks is composed — consists of silicaies of alumina. The alu- 

 mina itself in these several minerals varies from 11 to 38 per cent., but 

 generally averages about 20 per cent, of their entire weight. 



These silicates, when they occur alone, unmixed or uncombined with 

 other silicates, decompose very slowly by the action of the atmosphere. 

 They disintegrate, however, and fall to powder, when the alkaline sili- 

 cates with which they are associated in felspar, &c., are decomposed and 

 removed by atmospheric causes. In this way the deposits of porcelain 

 clay, so common in Cornwall and in other countries, have been pro- 

 duced from the disintegration of the felspathic rocks, and the clayey soils 

 wiiich occur in granite districts have not unfrequently had a similar origin. 



When contained in the soil, the silicates of alumina undergo a slow 

 decomposi\ion from the action of the various acid substances to which they 

 are exposed. A portion of their alumina is dissolved and separated by 

 these acids, and in this soluble state is either conveyed to the roots of 

 plants or is washed from the soil by the rains-^or by the waters that 

 arise from beneath. 



The ash of plants contains only a very small proportion of alumina, 

 yet even this small quantity they cannot derive from the silicates of this 

 substance, since these are all insoluble in water — as alumina itself is. 

 They obtain it, therefore, from some of those soluble compounds of alu- 

 mina of which I have spoken as being either occasionally present (pp. 

 204-5), or as being naturally formed in the soil. 



General remarks on these Silicates. — Of all these silicates it may be 

 remarked in general — 



1°. That besides existing in the minerals above-mentioned, and from 

 which they are conveyed into the soil, they are also slowly formed in the 



* I am indebted to Sir Charles Lemon for the analysis of a soil, on part of his own proper 

 ty, resting on serpentine, and bearing only Erica vagans, which illustrates the statement in 

 the text. This soil consists of silica 70, alumina with a trace of gypsum 20, oxide of iron 62, 

 and vegetable matter 3-8 percent. If this soil has been formed from the rock on which it 

 rests, the magnesia has been wholly washed out. Its constitution, however, points rather to 

 a decayed felspar or slate rock, as the source from which it has been derived. 



