Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/480

470 will appear fully determined by the other examples of this rock, which are described in the supplement to this communication. But this circumstance, already mentioned as being supposed to occur in Jura, does really appear to receive further confirmation here, since if the central ridge of Schihallien is surrounded by mica slate, a fact of which Mr. Palyfair entertains no doubt, and if this ridge is connected by any system of alternation with the sandstone of Glen Lyon, mica slate will appear to be a rock formed posteriorly to, or alternately with a rock of recomposed structure. Thus a “primitive” rock will be found to alternate with a “transition” one, an anomaly which either renders this distinction as useless as it is artificial, or compels us to modify the definition of transition rocks, or to form that total change of arrangement which I have more than once suggested with regard to the primitive and transition classes. It is, I trust, quite superfluous to say that it can have no title to the name of “granite,” with which it certainly does not possess any one common feature.

Having had an opportunity since I presented the foregoing memoir last year, of examining some other parts of Scotland where a similar rock occurs, I have thought it right to lay these supplementary observations before the Society, partly for the purpose of correcting the errors and supplying the defects of that communication, but principally for the sake of elucidating the history of a substance as yet but imperfectly understood, and but ill arranged, although it must already be obvious that it occupies considerable space among the older rocks, and is entitled to rank as a principal