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MR. C. BARRINGTON BROWN AND PROFESSOR J. W. JUDD 
most unalterable of substances, this is not the case with the mineral as it exists at 
considerable depths in the earth’s crust. 
9. The anhydrous oxide easily takes up water and recrystallizes as diaspore, 
and many examples of corundum, as suggested by Lawrence Smith and Genth, 
really consist of intimate admixtures of the anhydrous and hydrous oxides of 
aluminium. 
10. Unaltered corundum is, like quartz, a mineral without any true cleavage and 
breaks with a conchoidai fracture. 
11. The common partings parallel to the primitive rhombohedron which are so 
often found in corundum, are gliding planes produced by pressure. 
12. In addition to these partings, produced by mechanical means, there are others 
which are developed by chemical means, namely, hydration and union with silica and 
other oxides. This action tends to take place along definite crystalline planes— 
solution planes—which are parallel to the base and hexagonal prism. 
13. If, however, the crystal has been subjected to pressure, and had the rhorn- 
bohedral gliding planes developed in it, such planes become secondary-solution 
planes, along which chemical changes proceed very rapidly. 
14. By the formation of diaspore and the union of diaspore with silica and other 
oxides, the corundum gradually diminishes in hardness, lustre, and density; the 
changes sometimes appearing to go on in a perfectly capricious manner from the 
surface of the crystal, but more usually following either the primary solution planes, 
or, if the crystals have been subjected to mechanical change, the secondary solution 
planes. This change took place in some cases before, and in others after, the minerals 
were embedded in the limestone. 
15. The final result of these processes is to convert the corundum crystal into one 
of those pseudomorphs in hydrated silicates, damourite, margarite, chlorites, 
vermiculites, &c., which are so familiar to all mineralogists. 
Explanation of Plate 6. 
Note. —In describing the actual dimensions of the objects represented on this 
plate, the magnifying power of the objective is given as the numerator, and the 
reduction for the purposes of drawing as the denominator. The quotient, therefore, 
gives the actual linear enlargement. 
Pig. 1. Fine-grained pyroxene-gneiss, resembling a gabbro in its characters. It 
consists of plagioclase felspar, with a little quartz and orthoclase, the 
latter showing much of the “ quartz of corrosion of French authors. 
The crystals thus attacked are indicated by faint shading. The pyroxenes, 
which are present in moderate quantities, belong to both augite and 
