220 
MR. C. BARRINGTON BROWN AND PROFESSOR J. W. JUDD 
the accuracy of his conclusions.* As the result of his studies, Genth wrote, “ May 
it not be that the diaspore is so very finely distributed through the corundum, that 
even the best microscopic or other examination could not detect it, as I have just 
shown with regard to the admixture of corundum and spinel.”+ 
In many cases, alteration of corundum into diaspore can be seen, in thin sections 
under the microscope, to have taken place along the planes of secondary parting- 
following the rhombohedron. The reduction in specific gravity of the mineral serves 
as a measure of the hydration that has taken place. Thus, in an altered corundum 
from the United States, I found the specific gravity to be 3*88, while in a still more 
altered specimen from Bekwanna, Ceylon, given to me by Mr. C Barrington 
Brown, the density was as low as 3’79. As the density of unaltered corundum is 
4*02, and of diaspore is 3*32, we may calculate that the American specimen consists 
of four parts of corundum united with one part of diaspore; while the Ceylon 
specimen is made up of two parts of corundum with one of diaspore. In the last- 
mentioned specimens the presence of diaspore can be detected by the naked eye. The 
corundum crystals break easily along the rhombohedral parting-planes; and these 
are seen to be covered with films of diaspore, exhibiting its characteristic colour and 
lustre. Common corundum has an average specific gravity of 3*93, and may, there¬ 
fore, be regarded as a combination of six parts of corundum with one part of diaspore. 
It must be remembered, however, that in many cases the change has proceeded one 
step farther, and various silicates have been formed from the hydrated alumina. 
The hydration of corundum, and the union of the diaspore thus produced with 
silica and certain bases to form various silicates, is admirably illustrated in the case 
of the Burma examples. Scattered through the highly crystalline limestone we find 
a number of rubies, most of these being enveloped by a mass which clearly consists of 
the products of alteration of the crystallized aluminium oxide. In some cases the 
rubies are oidy surrounded by a thin shell of these alteration products, but in other 
instances every trace of the original ruby has disappeared, and the products of its 
decomposition fill the space once occupied by it. Under the microscope thin sections 
enable us to follow out the several stages of the metamorphism that has taken place. 
Immediately around the unaltered ruby there is always a layer, varying in thickness, 
of pure diaspore (hydrated alumina); but as we pass outwards we find this replaced 
by mixtures of various hydrated silicates, such as margarite, damourite, kaolin, &c., 
into which the diaspore insensibly passes (see Plate 6, figs. 10, 11, 12). The latter 
change is quite similar to that observed in the well-known case of the diaspore of 
Dilln, near Schemnitz, which has been investigated by A. Hutzelmann| and other 
authors. 
* ‘ Proc. Am. Pliil. Soe.,' vol. 20 (1882-3), pp. 381-392. See also, ‘Am. Jouru. Sci.,’ 3rd series, 
vol. 39 (1890), pp. 47-50. 
t ‘ Proc. Am. Phil. Soe vol. 13 (1873), p. 372. 
J ‘Bull. Freund, der Naturwiss.,’ Wien; ‘ Pogg. Ann.,’ vol. 28, p. 575; ‘Am. Journ. Sci.,’ 2nd ser., 
vol. 10 (1850), p. 247 ; also Berg u. Hiitten. Zeitsclir.,’ vol. 10 (1851), 11—12. 
