CHAPTER XLIII

THE BOSSES AND SHEETS OF GABBRO

Petrography of the Rocks--Relations of the Gabbros to the other members of the Volcanic series--Description of the Gabbro districts--Skye

In singular contrast to the nearly flat basalts of the plateaux, another series of rocks rises high and abruptly above these tablelands into groups of dome-shaped, conical, spiry, and rugged hills. It is these heights which, more than any other feature, relieve the monotony of the wide areas of almost horizontal stratification so characteristic of the volcanic region of the north-west. Their geological structure and history are much less obvious than those of the bedded basalts. Their mountainous forms at once suggest a wholly different origin. Some portions of them have even been compared with the oldest or Archæan rocks.[325] That they are really portions of the Tertiary volcanic series, and that they reveal a wholly distinct phase in the history of volcanic action, is now frankly admitted. Whether we regard them from the petrographical or structural point of view, they naturally arrange themselves into two well-defined groups. Of these one consists of highly basic compounds, of which olivine-gabbro is the most prominent. The other comprises numerous varieties--granite, granophyre, felsite, quartz-porphyry, pitchstone and others--all of them being more or less decidedly acid, and some of them markedly so. For reasons which will appear in the sequel, the former group must be considered as the older of the two, and it will therefore be described first.

[Footnote 325: This was my own first impression, when I began, as a boy, to ramble among them. The remarkable resemblance of some parts of them to ancient gneisses will be afterwards dwelt upon. Macculloch had correctly grouped them with the other overlying rocks, and this conclusion was afterwards confirmed by Prof. Zirkel.]