The Caernarvonshire volcanic area furnishes another admirable example of the intrusion of basic sills as a final phase of eruptivity. These masses have been carefully separated out on the maps of the Geological Survey, which present a striking picture of their distribution and their relation to the other igneous rocks. An examination of the maps shows at once that the basic sheets tend to lie parallel with the bedding along certain horizons. In the southern and western portions of the area they have forced themselves among the Lower Silurian sedimentary strata that underlie the Bala volcanic group--a position analogous to that taken by the corresponding sills of the Arenig series. But they likewise invade the volcanic group itself. Along the eastern borders of the district they abound, especially in the higher parts of the volcanic pile, where they have been injected between the flows, and have subsequently participated in the abundant plication of the rocks between the mountains and the line of the River Conway. The curvatures into which the rocks of the region have been thrown, and the consequent breadth of country over which the volcanic sheets can now be examined, furnish a much better field than Merionethshire for the attempt to trace the probable centre or centres from which the basic magma of the sills was protruded. A study of the Survey maps soon leads to a conviction that the intrusions were not connected, except perhaps to a trifling extent, with the great line of western vents. It is remarkable that the older strata which emerge from under the volcanic group on its western outcrop are, on the whole, singularly free from sills, though some conspicuous examples are shown opposite to Mynydd-mawr, while a few more occur further north along the same line. Their lenticular forms, their short outcrops, and their appearance on different horizons at widely separated points seem to indicate that the sills probably proceeded from many distinct subterranean pipes. Their greater abundance along the eastern part of the district may be taken to indicate that the ducts lay for the most part considerably to the eastward of the line of western vents. They may have risen in minor funnels, like that of Capel Curig. It is noteworthy that so abundant an extravasation of basic material should have taken place without the formation of numerous dykes. We have here a repetition of the phenomena that distinguished the preceding Arenig volcanic period in Merionethshire, and it will be remembered that the Llandeilo eruptions of Builth were likewise followed by the injection of large bodies of basic rock. As an enormous amount of igneous magma may thus be impelled into the Earth's crust without the formation of dykes, it is evident that the conditions for the production of sills must be in some important respects different from those required for dykes.