That this zone of "greenstone" sills belongs to the period of the Merionethshire volcanoes may be reasonably concluded. The way in which they follow the line of the great escarpment, their almost entire absence from the Cambrian dome to the west, their cessation as the overlying lavas and tuffs die out laterally, and their scarcity above the lower part of the volcanic group, seem to indicate their close relationship to that group. Moreover, that they must have been as a whole later than the main part of the lavas and tuffs may be inferred from their position. The molten material of which they were formed could hardly have forced its way between and across the strata unless egress to the surface had been impeded by some thick overlying mass. The "greenstones" may therefore be regarded as lateral emanations from funnels of more basic lava towards the close of the volcanic period. Possibly some at least of the highly slaggy and vesicular bands to which I have referred may represent portions of this material, which actually flowed out as streams of lava at the surface. But there is likewise evidence of extensive intrusion of more siliceous rocks. On the Geological Survey map, besides the numerous "greenstones," various sheets of "felspathic porphyry" are represented as running with the general strike of the region, but here and there breaking across it. One of the most remarkable of these acid sills is that which, in the noble precipice of Cader Idris, has a thickness of about 1500 feet and a length of three or four miles. It is shown on the map to be transgressive across other rocks, and, as seen on the ground, it maintains the uniformity of texture which is characteristic rather of sheets that have solidified underneath than of those which have congealed with comparative rapidity at the surface. On a fresh fracture the rock presents a pale bluish-grey tint, becoming yellowish or brownish as the result of weathering. Its texture is finely granular, with occasional disseminated felspars. Under the microscope a section of it was found by Dr. Hatch to exhibit the characteristic structure of a microgranite, a confused holocrystalline aggregate of quartz and felspar, with a few porphyritic felspars. Messrs. Cole and Jennings have proposed to revive for this rock Daubuisson's name "Eurite."[158] [Footnote 158: Mr. Harker speaks of the rock as a granophyre.] A similar rock occurs at a lower horizon among the Lingula Flags at Gelli-llwyd-fawr, two miles south-west of Dolgelly,[159] and much microgranite has been injected along the slopes above Tyddyn-mawr. [Footnote 159: Messrs. Cole and Jennings, _op. cit._ p. 435.]