220 PREVAILING THEORY CONCERNING [Ch. XI. termed lava where it issues on the flanks of the mountain: and we must therefore consider the terms trap and volcanic as synonymous. Again, the difficulty of distinguishing the volcanic from the Plutonic rocks is sufficiently great; for we must draw an arbitrary line between them, — there being an insensible passage from the most common forms of granite into trap, as detailed by Dr. Macculloch in his geological descriptions of Aberdeenshire, and other parts of Scotland. It would be easy to multiply examples to prove that the granitic and trap-rocks pass into each other, and are merely different forms which the same elements have assumed, according to the different circumstances under which they have con solidated from a state of fusion. The same lava which is porous where it has flowed over from the crater, and where it has cooled rapidly and under comparatively slight pressure, is compact and porphyritic in the dike. Now, these dikes are evidently the channels of communication between the crater and the volcanic foci below; so that we may suppose them to be continuous to the depth of several hundred fathoms, or perhaps two or three miles, or even more; and the fluid matter below, which cools and consolidates under so enormous a pressure, may be supposed to acquire a very dis tinct texture, and become granite. For these reasons, it is probable that Plutonic rocks have originated in the nether parts of the earth’s crust, as often as the volcanic have been generated at the surface. And we may also infer, that during each preceding period, whether tertiary or secondary, there have been granites and gra- nitiform rocks generated, because we have already discovered the monuments of ancient volcanic eruptions at almost every period. This supposition is corroborated by the observations of several geologists, that granite is not confined to one par ticular period, antecedent to the introduction of organic remains into our planet; but has been produced again and