DURING THE TERTIARY" PERIOD IN THE BRITISH ISLES. 159 



abundant, cutting through all the other rocks (fig. 50). So numerous are they that the 

 geologist ceases to take note of them when his thoughts are engaged upon the problems 

 presented by the masses through which they rise. 



b. Small Isles. — In the island of Eigg three small bosses or sheets occur. That 

 at the northern end rises through the Jurassic sedimentary rocks, and forms a bold cliff 

 from 150 to 200 feet high. It is a light grey granophyric porphyry, with rounded blebs 

 of quartz in a micropegmatic base of quartz and felspar. The other two masses, of 

 smaller size, cut through the bedded basalts.* 



In the opposite island of Eum, the acid protrusions play a much more important part. 

 On the east side of the hills, they occur in sheets at the base of the gabbros; on the west 

 side, they form a large tract of hilly ground, which, stretching along the coast line for 

 about three and a half miles from the headland of A' Bhrideanaeh to Harris, forms there 

 a range of shattered sea-cliffs, parts of which tower for 1000 feet above the Atlantic 

 breakers that beat about their base. The area extends inland to the slopes on the west 

 side of Loch Sgathaig, a distance of about three and a half miles, descending in a range 

 of precipices along its northern front, and reaching in its culminating summit, Orval, a 



y 



d a 



Fig. 51.— Section on north side of Orval, Rum. a, Cambrian sandstones; b, bedded basalts of Fionn Chro ; 



c, dolerite ; d, quartz-porphyry. 



height of 1868 feet above the sea. The rocks of which this triangular area consists 

 resemble those of the Mull bosses. They are chiefly quartz-porphyries, becoming f elsitic 

 in texture towards their contact with adjacent rocks. In some places, as was noticed by 

 Macculloch on the sea-cliffs,t they have a rudely bedded structure. Thus on the north- 

 west front of Orval, this structure is shown by parallel planes that dip outwards or NW. 

 at 30° to 40°, and which are made still more distinct by an occasional intrusive dyke or 

 sheet of basalt between their surfaces. I shall again have occasion to refer to the 

 internal arrangement of the granitoid bosses, in the account of those of Skye. 



Like the gabbros already noticed, the granophyres, porphyries, and felsites of Eum 

 have been intruded at the base of the volcanic series, and over much, if not all, of their 

 area lie directly on the red Cambrian (Torridon) sandstone. That the bedded basalts 

 once covered them is shown by the position of the three outliers of the basalt-plateau 

 already noticed. But a fourth outlier still lies upon the porphyry of Orval as a cake that 

 dips gently northward. It consists of a bedded, dark, finely crystalline, ophitic dolerite, 

 porphyritic in places, with a rudely prismatic or columnar structure (fig. 51). It has 

 undergone contact metamorphism, and tongues from the underlying rock project up 



* Quart. Jour. Geol. Soc, xxvii. (1871) p. 294. t Western Islands, vol. i. p. 487. 



