1871.] 



GEIKIE lEETIAEY VOLCANIC EOCKS. 



305 



It has a minutely granular texture, and is usually strongly por- 

 phyritic, 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 l 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 gra- 

 nules 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 



* The notes given above of the microscopic structure of the Eigg pitchstones 

 are the results of merely a preliminary examination. I hope to be able even- 

 tually to form materials for an essajf on the minute structure of the pitchstones 

 of Scotland. 



