ON THE RUBIES OF BURMA AND ASSOCIATE!) MINERALS. 209 
rubies and spinels, but come from another locality, their parent rock being a gneiss 
or schist. The significance of these facts will become apparent when we consider 
the association of minerals at other localities where corundum is found in limestone, 
as at St. Gothard, in Switzerland, and in the limestone belt of Orange Co., N.Y., 
and Sussex Co., N.J., in the United States. 
There are two kinds of evidence which are available in determining the nature 
of the minerals associated with the ruby and spinel in Burma. The first and most 
satisfactory is derived from examination of the limestones which contain the gems. 
This may be done either by means of thin sections, or by dissolving the limestone in 
acid and separating and determining the various insoluble minerals left behind. 
Nature, however, has done this latter kind of work for us on a grand scale, and in the 
alluvial deposits we find great accumulations of the insoluble minerals which were 
originally embedded in the limestone, and here we naturally obtain larger and finer 
crystals than we can hope to procure by carrying on the work of solution in the 
laboratory. It must not be forgotten, however, that there is an element of uncertainty 
in the evidence afforded by these alluvial deposits, as some of the minerals found 
in them may be derived, not from the corundiferous limestones, but from other asso¬ 
ciated rocks in the district; hence it will not be wise to accept the evidence afforded 
by the alluvial deposits, unless it is corroborated by the fact of the same minerals 
being actually found in the limestone or in the residues obtained by its solution. 
In the following list are included all the species of minerals which I have been 
able to determine in the crystalline limestone of Mogok and the surrounding districts 
and in the granular limestone of Sagyin. In the remarks about the several minerals 
I have, however, often availed myself of information derived from the study of the 
finer and larger specimens yielded by the alluvial deposits. 
1. Corundum .—This mineral rarely presents crystalline outlines, for, as we shall 
presently see, chemical disintegration and alteration into diaspore and various 
silicates has gone on from the exterior, and the core of unaltered aluminium oxide 
left behind is of irregular form. Occasionally, however, well defined crystals of 
corundum are obtained, and it is a noteworthy circumstance that, as has been already 
pointed out by Mr. F. R. Mallet, the Burma rubies generally assume peculiar and 
characteristic combinations of forms. # Crystals made up of the rhombohedron and 
basal plane (R and OB : 111, 100) are very common, though the prism (oo P2, 101) 
occasionally enters into the combination. Forms in which the prism and pyramids 
(so constantly seen in the corundums from Ce} r lon, Southern India, United States, &c.) 
are found predominating, are, so far as my observation goes, very rare ; though several 
such combinations are figured by Mr. Mallet. 
The forms present in the ruby-crystals of Burma are thus seen to be the same as 
those found in the artificial rubies made by MM. Fremy and Feil ; but in the arti- 
* ‘ A Manual of the Geology of India,’ Part IV., Mineralogy (18S7), p. 43, Plate 1, figs. 6, 7, 8. 
MDCCCXCVI.—A. 2 E 
