Dear Professor Balfour,—In vol. xvi. of the “Transactions of the Royal Society of Edinburgh,” I have described and given drawings of the pressure cavities which I discovered in topaz; and in vol. xxiii., just published, I have pointed out the geological relations of these cavities.

As the specimens of topaz containing them are so rare that I have found only five out of many hundreds which I have examined;— as the existence of such cavities with a polarising structure around them, proving that the topaz was in a soft or plastic state, will hardly be admitted by those who believe that the topaz was formed by aqueous deposition; —and as it is quite possible that other specimens containing such very minute cavities may never be found, even when diligently searched for, I think it right to present to the Society for preservation the five topazes in which the cavities were found.—I am, ever most truly yours.