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 Hat 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 Archaean rocks. 1 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 petrograplrical 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. 
i. PETROGRAPHY OF THE GABBRO AREAS 
Since the publications of Macculloch, the occurrence of beautiful 
varieties of highly basic rocks among the igneous masses of the W estern 
Isles has been familiar to geologists. They were named by him “ hyper- 
sthene rock ” and “ augite rock,” 2 names which continued in use until 1871, 
1 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. 2 Western Islands , vol. i. pp. 385, 484. 
