Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/80

 hardened it is known by the name of compact felspar. As occur in this simple state they must be considered mineralogically as examples of these different substances, although in a geological sense we cannot without troublesome circumlocution and great confusion consider them under any but the general term already adopted. The colour of this base varies from ochrey yellow and dirty flesh colour to gray: it is often cavernous and filled with a ferruginous clay. In other situations it contains crystals of felspar, either of the same or of a different colour, and thus forms various kinds of porphyry. The predominant form however is that whence its name has been imposed, an aggregate of felspar and hornblende, in which the hornblende generally bears a very small proportion to the other ingredient: the porphyritic character is sometimes added to this mixture. In some rare instances quartz enters into its composition, and in such instances it trenches near upon the syenitic granite, a distinction concerning which I have spoken. in the paper on Glen Tilt, to be found in this volume. More rarely still contains mica, and in this case it becomes utterly impossible to distinguish it from those granites which contain crystals of hornblende superadded to the usual threefold mixture of quartz, felspar and mica. Under such circumstances it is quite conceivable that specimens should be met with from which the hornblende was absent, since even in those I have described, it is very thinly scattered through the mass. In such a case, should it occur, mineralogy, unassisted by geological observation, would tend to mislead as in reasoning respecting its position, and we are thus driven to acknowledge, in geological description, the necessity of superadding to mineral characters an accurate knowledge of the connections of a rock respecting which we are reasoning: I must therefore, from a geological knowledge of