In a paper read before the Geological Society of London, in 1889, I contended that the biotite of the gneisses and schists of Malvern had been produced out of hornblende through the intermediate form of chlorite. As this mineral change is of capital importance in explaining the origin of the Malvern schists, I made it one of the principal topics of the paper, and I trusted that the evidence I offered in support of it would be considered adequate. The further communication on the Malvern rocks, which I read on April 26th of the present year, assumed the adequacy of this proof. I found, however, that in certain quarters some scepticism on the point still existed, and it was suggested that I had overlooked some of the chemical difficulties which seemed to forbid the acceptance of my view. I therefore crave permission to reply to this objection in these pages ; and I desire, at the same time, to point out that since the reading of my paper in 1889, the production of biotite from chlorite has been affirmed by several writers of admitted authority.