In the introductory portion of the paper a sketch is given of the present state of opinion among Continental petrographers regarding volcanic rocks associated with palæozoic formations. The author points out that a relic of Werner's belief in the recentness of volcanic action may still be traced pervading the ideas of German geologists. In this paper he endeavours to show that, alike in their formation in the field and in their structure under the microscope, the basalts and tuffs of palæozoic time were as truly the products of volcanic action as the lavas and ashes of the present day.