224 
MR. C. BARRINGTON BROWN AND PROFESSOR J. W. JUDD 
The hydrates and silicates have in some cases been produced along more or less irregular 
depressions, but these have shown such a general tendency to follow definite directions 
within the crystal that the result has been a mass built up of granules, each 
having an approximately crystalline form. Fig. 21 shows a crystal of black spinel 
(ceylanite), the summit magnified 2 diameters, the faces of which show etched figures, 
but at one anode this etching has gone so far as to leave a number of octahedral 
figures standing out in relief. In fig. 22 (also magnified 2 diameters) of a purple 
spinel from Burma the whole crystal is found broken up in the same manner, into an 
aggregate of small rhombohedral polysynthetic crystal of some authors. These 
resultant masses resemble in a striking way the models built up of “ fundamental 
forms ” by which Hauy and other crystallographers have sought to illustrate their 
ideas of crystals architecture. In the case of the spinel, the form of the granules into 
which the large crystals break up, is the regular octahedron. A very large and 
remarkable specimen of spinel from Burma, in the possession of Mr. Streeter, 
exhibits this peculiarity in the same striking manner as the smaller ones figured. 
Beginning with natural etched figures (Verwitterungsfiguren), we find in these cases 
that the depressions become deeper and deeper till the whole crystal is reduced to a 
skeleton-like mass. 
[A very interesting problem which suggests itself to the student of these remark¬ 
able changes in the corundum and spinel, is that of the time and place at which they 
must have taken place. That, at ordinary temperatures and pressures, both 
corundum and spinel are very slowly, if at all, acted upon by atmospheric agencies, 
is shown by the condition of the specimens found in alluvial deposits. In spite of the 
extreme hardness of the gems, they may be found to have suffered greatly from 
mechanical attrition while exhibiting little if any evidence of chemical change. 
The hydration of the oxides and their union with silica must have been brought 
about, therefore, while they still constituted portions of deep-seated masses, and were 
