Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/445

 It has a minutely granular texture, and is usually strongly porphyritic, with crystals of orthoclase, sometimes a quarter of an inch in length.

That portion of the mass which forms the eastern end, or Scur proper, shows under microscopical examination a much less perfect glass than any of the veins above described. With a 1/5 object- glass the rock seems to be made up of a confused aggregate of short pale fibres or hairs matted together. These are much more minute, and proportionally thicker than the hair-like bodies in the veins, and they are so abundant as to form apparently the whole or nearly the whole of the rock. At the opposite extremity of the ridge, the rock of Beinn Bhreac is less porphyritic. Examined with the microscope it shows a similar, but rather coarser texture, through which, in addition to the orthoclase, there are diffused small crystals of a delicately striated felspar*.

The grey porphyry, which occurs in beds and forms a subordinate part of the mass of the Scur ridge, is usually a somewhat decomposed rock. Where a fresh fracture is obtained it shows a fine-grained, sometimes almost flinty, grey felspar base containing clear granules of quartz, and facets of a glassy felspar, probably orthoclase. In some places the rock is strongly porphyritic. Although the line

Fig. 9. — Section at the base of the Scur of Eigg (east end).

of separation between this porphyry and the pitchstone is usually well defined, it is sometimes so obscure, and the two rocks so shade

are the results of merely a preliminary examination. I hope to be able eventually to form materials for an essay on the minute structure of the pitchstones of Scotland.
 * The notes given above of the microscopic structure of the Eigg pitchstones