I was not present when Dr. Callaway's last paper on the Malvern Rocks was read before the Geological Society, and had not the Opportunity of joining in the discussion that followed the reading of the paper.The author has now given his views regarding the “Conversion of Chlorite into Biotite;” in a condensed form in the GEOLOGICAL MAGAZINE (Dec. 1893, p. 535), and has expressly invited criticism on them; I venture, therefore,inresponse to this invitation, to give expression to a difficulty that presents itself to my mind.

It is not necessary to enter into a discussion of the authorities quoted by Dr. Callaway; but I remark in passing that two of the papers referred to prefess to deal with cases of contact metamorphism, and consequently the conditions that must have prevailedinthose cases are altogether different from those which govern Dr. Callaway's supposed case of dynamo-metamorphism.incases of contact action one can readily understand how aqueous acid vapours, or liquids, emanating from the molten igneous rock under high pressure penetrated the adjoining rocks, and carried with them in solution some of the constituents of the igneous magma.