At this period but little had been done in studying the chemical characters of aqueous and igneous rock-masses respectively; and while, on the one hand, the close similarity in chemical composition between the ancient basalts and many modern lavas was not recognised, the marked distinction between the composition of such materials and most aqueous sediments remained, on the other hand, equally unknown. Nor had anything been yet accomplished in the direction of the study of rock-masses by the aid of the microscope. Hence there could be no appeal to those numerous structural peculiarities that at once enable us to distinguish the most crystalline aqueous rocks from the materials of igneous origin. On the other hand, there undoubtedly exist rocks of a black colour and crystalline structure, sometimes presenting a striking similarity in general appearance to the basalts, which contain fossils and are undoubtedly of aqueous origin. Thus on the shore near Portrush, in the North of Ireland, and in the skerries which lie off that coast, there occur great rock-masses, some of which undoubtedly agree with basalt in all their characters, while others are dark-coloured and crystalline, and are frequently crowded with _Ammonites_ and other fossils. We now know that the explanation of these facts is as follows. Near where the town of Portrush is now situated, a volcanic vent was opened in Miocene times through rocks of Lias shale. From this igneous centre, sheets and dykes of basaltic lava were given off, and in consequence of their contact with these masses of lava, the Lias shales were baked and altered, and assumed a crystalline character, though the traces of the fossils contained in them were not altogether obliterated. In the last century the methods which had been devised for the discrimination of rocks were so imperfect that no distinction was recognised between the true basalt and the altered shale, and specimens of the latter containing _Ammonites_ found their way to almost every museum in Europe, and were used as illustrations of the 'origin of basalt by aqueous precipitation.'