It is a common belief that the filling in of the steam-cavities has taken place long subsequent to the volcanic period, by the slow percolation of meteoric water through the rock. I believe, however, that at least in some cases, if not in all, the conversion of the vesicular lavas into amygdaloids was effected during the volcanic period. Thus it can be shown that the basalts which have been disrupted by the gabbros and granophyres were already amygdaloids before these basic intrusions disturbed them, for the kernels of calcite, zeolite, etc., have shared in the general metamorphism induced in the enclosing rock. Again, the blocks of amygdaloid contained in the agglomerates of the volcanic series are in every respect like the amygdaloidal lavas of the plateaux. It would thus seem that the infilling of the cavities with mineral secretions was not merely a long secular process of infiltration from the cool atmosphere, but was more rapidly completed by the operation of warmer water, either supplied from volcanic sources or heated by the still high temperature of the cellular lavas into which it descended from the surface.[223] [Footnote 223: Professor J. D. Dana, originally an advocate of infiltration from above, subsequently supported the view that the kernels of amygdaloids were filled in by the action of moisture within the rocks during the time of cooling.--_Amer. Journ. Sci._ ser. 3, vol. xx. (1880), p. 331. Messrs. Harker and Marr have demonstrated that the Lower Silurian vesicular lavas of the Lake district had already become amygdaloids before the uprise of the Shap granite.--_Quart. Journ. Geol. Soc._ vol. xlix. (1893).] 4th. Banded or stratiform lavas, consisting of successive parallel layers or bands which weather into projecting ribs and flutings. The deceptive resemblance to sedimentary rocks thus produced has no doubt frequently led to these lavas being mistaken for tuffs. As I have recently found them to be much more plentiful than I had supposed, a more detailed description of them seems to be required. The banded character arises from marked distinctions in the texture of different layers of a lava-sheet. In some cases (_a_) these distinctions arise from differences in the size of the crystals or in the disposition of the component minerals of the rock; in others (_b_) from the varying number and size of the vesicles, which may be large or abundantly crowded together in some layers, and small or only sparsely developed in others. The structure thus points to original conditions of the lava at the time of its emission and may be regarded as, to some extent, a kind of flow-structure on a large scale.