In the last number of the Geological Magazine there is an abstract from an interesting and important paper by Prof. Hull and Mr. Traill on the relative ages of certain igneous rocks of Co. Down, Ireland. In that paper there is one paragraph on which I should like to offer a few remarks, as it refers to a previous communication from myself, and relates to a subject in which I take a special interest. The paragraph is as follows:—“It might have been supposed that microscopical examination would show some distinction in the basalts of these geological ages, but recent investigations by Zirkel, D. Forbes, Allport, and others, tend to show that there is no criterion of age amongst the constituents of basalt, dolerite, or melaphyre.”