These acid rocks have been regarded by some observers as intrusive sheets, by others as lava-streams that were poured out at the surface. If account be taken simply of their petrographical characters, they find their nearest analogies among the intrusive quartz-porphyries of older geological periods. The presence of flow-structure in them has been thought to indicate that they were superficial streams. But this structure may be found in dykes and intrusive sheets as perfectly as in lava-flows, so that it cannot by itself be taken as proof of a surface-discharge of lava. It must be confessed that, both in the main mass of quartz-porphyry and in the abundant fragments of it in the overlying conglomerates and breccias, there is an absence of such scoriform portions as one would naturally look for in a superficial lava-stream;[103] while, on the other hand, the rock generally presents the tolerably uniform flinty texture so familiar in intrusive sheets of similar material.[104] My own impression is that these igneous masses were probably erupted to the surface as long banks which rose above the waves; that they were thus exposed to prolonged subærial and marine denudation; that by this means any upper more cellular portions of the lava which may have existed were broken up and pounded down into detritus, and thus that what is now visible is a part of the eruptive rock which originally lay at some depth within its body. This view is confirmed by a study of other lavas which are found on different platforms in the detrital deposits that overlie the Llyn Padarn quartz-porphyry. [Footnote 103: But the Llyn Padarn rock, like many acid lavas, may never have possessed a vesicular structure in any portion of its mass. The sheets of felsite which occur among the overlying tuffs are not cellular, but present the closest resemblance to the main mass below.] [Footnote 104: Mr. Blake brought forward the evidence of a section on the north or under side of the Llyn Padarn ridge to show that the rock has there been intruded into the Cambrian strata (_Quart. Journ. Geol. Soc._ vol. xliv. (1888), p. 283). But the rock supposed by him to be altered slate has been shown to be intrusive "greenstone" (Miss Raisin, _op. cit._ vol. xlvii. (1891), p. 336).]