The trap-rocks seen there exhibit a wide range of structure, texture, colour, and general aspect. There are two pretty marked kinds--the augitic or hornblendic, and the felspathic; the former being usually of a more or less crystalline aspect; the latter, commonly dull, and often without any crystals.[75] In the augitic traps, the crystals are sometimes of large size and well-defined, so that the rock could hardly be distinguished at first sight from an ordinary grey granite, while at other times, and not unfrequently even in other portions of the same mass, the stone assumes a black appearance without distinct crystals. The former variety would be called a _greenstone_, the latter a _basalt_; the chief components in either case being felspar and hornblende, or felspar and augite, with a variable admixture of other minerals, the shade of colour varying from a pale blue or leek-green, through the different hues of grey, to a deep velvet black. There are other traps, however, consisting entirely, or nearly so, of felspar, whence they are known as _felstones_. Such rocks enjoy a wide range of colour, some of them being pure white, others of a bluish grey or dingy brown; and they may be seen graduating from a pale yellow, or flesh-colour, to a brick-red or deep purple. When a trap displays distinct disseminated crystals, usually of felspar, it becomes a _porphyry_; when it shows rounded cavities, like those of furnace-slag, it is said to be _vesicular_; and when these globular or almond-shaped cavities are filled with carbonate of lime, chalcedony, or other minerals, the rock forms an _amygdaloid_. Such peculiarities of structure indicate to some extent the origin of the mass, and may be found in any kind of trap. Thus we have porphyritic greenstones, basalts, or felstones, and the same rocks may be likewise vesicular or amygdaloidal. Some of them, such as many greenstones, display on weathered surfaces that curious spheroidal structure already alluded to; others are built up into geometric columns. [Footnote 75: This distinction, though a sufficiently safe one in some localities, must not be held as by any means universal in its application, the felspathic traps being often as crystalline in aspect as the augitic, and the augitic, on the other hand, as dull as the felspathic.] Such peculiarities of composition and structure form the basis of a mineralogical classification of the igneous rocks, which is of use in working out the geology of a district. The most convenient subdivision for our present purpose, however, is that which proceeds upon the origin and mode of occurrence of the trap-rocks. Viewed thus, they resolve themselves into two great groups, the _intrusive_ and _contemporaneous_, both of which contain greenstones, basalts, &c.,--the sole distinction between those of the one class and those of the other, being the relation of age and mode of occurrence which they bear to the surrounding rocks.