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The sandy covering sometimes fills up furrows in the gravel, as does the gravel in the underlying tertiary formations. The pebbles are frequently whitewashed and even cemented together by a peculiar, white, friable, aluminous, fusible substance, which I believe is decomposed feldspar.
I appeal to the analogy which, in this treatise, he has formed, between the stalactical concretions upon the surface of the earth, and the mineral concretions of siliceous substance. As an example of the great lights, and penetrating genius, of this assiduous studier of nature, I refer to the judicious observations which he has made upon the subject of aluminous earth, in this dissertation.
It may also be inquired whether this aluminous slate be a transition-formation lying on the primitive mica-slate of Araya, or whether it owe its origin merely to a change of composition and texture in the beds of mica-slate.
Among the lowest species of this combustible schistus are those argillaceous strata in Yorkshire from whence they procure alum in burning great heaps of this stone, which also contains sulphur, to impregnate the aluminous earth with its acid.
There is also a little Moor in Hertfordshire, thro' which a Water runs that frequently gives the Passant Horses that drink of it, the Colick or Gripes, by means of the aluminous sharp Particles of its Earth; Its Air is also so bad, as has obliged several to remove from its Situation for their Healths: The Dominion of the Air is likewise so powerful over Vegetables, that what will grow in one Place won't in another, as is plain from the Beech and Black Cherry Tree, that refuse the Vale of Ailesbury tho' on some Hills there, yet will thrive in the Chiltern or Hilly Country: So the Limes and other Trees about London are all generally black-barked, while those in the Country are most of them of a Silver white.
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