Trap-rocks generally may be variously classified according to the aspect under which we view them. Mineralogically they are _augitic_, when the mineral _augite_ enters largely into their composition; _hornblendic_, when the _augite_ is replaced by _hornblende_; and _felspathic_, where _felspar_ forms the most marked constituent. The first class includes all the dark homogeneous compounds called _basalts_; the second, the hornblendic _greenstones_, or _diorites_; and the third, the _felstones_, _porphyries_, and _tufas_. Geologically, they are _beds_ when they are interstratified with the contiguous rocks; and _dykes_ or _veins_ when they penetrate them like walls, or in an irregular manner. The former class may be either of the same age with the rocks among which they lie, or of a later date, just as in a pile of books the centre one may either have been placed there originally with the rest, or thrust in long afterwards. The latter class must always be later than the rocks which they traverse, for it is plain the rocks must have been in existence before trap-dykes and veins could be shot through them. Hence geologists are accustomed to speak of contemporaneous and subsequent trap-rocks: the one list including all the tufas, and those melted rocks which can be shown to have been erupted during the time when the limestones, sandstones, or shales around them were forming; the other embracing all the dykes and veins along with those beds of melted rock which have been intruded between the strata. These and other distinctions will be better understood from a few examples collected chiefly from the carboniferous district of central Scotland.