The rocks whose microscopic structure it described in this paper were collected by my friend, M. G. Stuart, Esq., during a tour in Australia, under the kind auspices of A. W. Howitt, Esq., F.G.S.,the well-known geologist, who has so largely added to our knowledge of one of the most interesting districts of that continent. Some of them have been already noticed by that gentleman, but as the volumes in which his descriptions are published are not very accessible to English readers, and as two of them are rocks to which I have devoted special attention, I have ventured to bring them to the notice of our Society.