III. Notes on alteration induced by heat in certain vitreous rocks; based on the experiments of Douglas Herman, F. I. C., F. C. S., and G. F. Rodwell, late science master in Marlborough college

In this paper an endeavour is made to show the nature of the changes which have resulted from the action of heat upon certain vitreous rocks. The changes which take place in such rocks through natural processes may sometimes be effected by heat alone, at others by heat in presence of moisture. Of these actions the latter is probably the more frequent, but, at the outset, it seems important to ascertain the action simply of dry heat before studying the more complicated conditions engendered by the presence of water and the pressure of superincumbent rock masses.