2. In some districts the dykes can be connected with the gabbros which occur as intrusive sills and irregular bosses in the basalt-plateaux and among older rocks. The gabbros, however, are traversed by still later dykes, which must then be independent of any visible mass of these rocks. The connection of dykes with the gabbros is what we might naturally expect to find, if the more coarsely crystalline rock represents portions of the basic magma which consolidated at some depth below the surface. If we could penetrate deep enough, it is not improbable that the dykes might be found in large measure to shade downward into vast bodies of gabbro. Such a relation has been observed in the Yellowstone district, where Mr. Iddings has noticed that the centre toward which the dykes of the Old Crandale volcano converge is a large mass of granular gabbro, passing into diorite, the dykes becoming rapidly coarser in grain as they approach the gabbro-core.[216] [Footnote 216: _Journ. Geol._ i. (1893), p. 608.] 3. The rise of molten rock in thousands of fissures over so wide a region is to my mind by far the most wonderful feature in the history of volcanic action in Britain. The great plateaux of basalt, and the mountainous bosses of rock by which they have been disrupted, are undoubtedly the most obvious memorials of Tertiary volcanism. But, after all, they are merely fragments restricted to limited districts. The dykes, however, reveal to us the extraordinary fact that, at a period so recent as older Tertiary time, there lay underneath the area of Britain a reservoir or series of reservoirs of lava, the united extent of which must have exceeded 40,000 square miles. That the material of the dykes rose in general directly from below, and was not, except locally, injected laterally along the open fissures, may be inferred, although proof of such lateral injection on a small scale may here and there be detected. The narrowness of the rents, and their enormous relative length, make it physically impossible that molten rock could have moved along them for more than short distances. The usual homogeneous character of the dyke-rocks, the remarkable scarcity of any broken-up consolidated fragments of them immersed in a matrix of different grain, the general uniformity of composition and structure from one end of a long dyke to another, the spherical form of the amygdales, the usual paucity of fragments from the fissure walls--all point to a quiet welling of the lava upward. Over the whole of the region traversed by the dykes, from the hills of Yorkshire and Lancashire to the remotest Hebrides, molten rock must have lain at a depth, which, in one case, we know to have exceeded three miles, and which was probably everywhere considerably greater than that limit.