Vol.62.] STKTJCTUEE OF THE SGTJKB OP EIGG. 65 



sheet, falls naturally into its place as a member of this group of 

 intrusions, with which it has much in common. It further resem- 

 bles the rest in the absence of steam-vesicles, a negative character 

 which is not easily reconcilable with a glassy subaerial lava con- 

 taining 2'7 per cent, of water. The irregularly undulating lower 

 surface of the sheet also agrees with what is seen in many other 

 members of the group which have assumed the stratiform habit. 

 Indeed, it is very noticeable that the remarkable regularity observable 

 in the basic sills of the region is rarely realized in the sheet-formed 

 intrusions of more acid rocks. 



The history of Tertiary igneous activity within the British area 

 is, in its main divisions, a simple one. Setting aside the Sgurr 

 of Eigg, all the superficial volcanic manifestations, including the 

 general outpouring of basic lavas and the more varied local out- 

 bursts at certain centres, were comprised within the earlier portion 

 of the time. Later, the various groups of plutonic rocks— ultra- 

 basic, basic, and acid— were intruded in order. Later still came the 

 long succession of minor intrusions — sills and dykes — embracing 

 numerous episodes, and doubtless representing a prolonged lapse of 

 time. This general sequence of events in a complete cycle of igneous 

 activity may be paralleled in various other regions. If the pitch- 

 stone of the Sgurr of Eigg, belonging to the very latest epoch, is 

 held to represent a belated volcanic outpouring, it must stand as a 

 unique exception. Without setting up such considerations against 

 direct evidence, we may welcome any well-grounded interpretation 

 which brings the apparent anomaly into harmony with what seems 

 to be a significant law. 



One other point remains -to be noticed in conclusion. The 

 picture of the pitchstone-lava of the Sgurr filling an old river-valley 

 has excited general interest, chiefly as affording a striking monument 

 of erosion. Now, the unequivocal proof of enormous erosion which 

 meets the eye in every part of the Inner Hebrides is, on any 

 reading of the phenomena, sufficiently impressive. Such sections 

 as that of Beinn M6r in Mull prove that, in some parts of the 

 region at least, the alternations of basalts and dolerites have had 

 an aggregate thickness of more than 3000 feet. Assuming, as is 

 generally supposed, that sill-intrusions can be formed only under a 

 considerable cover, the alternating succession must have been 

 capped originally by some thickness of basalt-lavas free from sills, 

 adding perhaps another thousand feet to the total. Comparing 

 this with the existing relief and geological constitution of the isles, 

 we see reason to allow extraordinary activity to the agents of 

 erosion in this region. In some parts, as on Bum and over about 

 one-half of Skye, the great thickness of basic rocks has been 

 entirely removed, and the plutonic masses, which in a general 

 sense underlay them, have been left standing out to heights of 

 2500 and 3000 feet. This indicates a vertical erosion amounting, 

 in some places and over considerable areas, to perhaps 5000 feet. 

 The actual removal of material has, however, been vastly greater 



Q. J. G. S. No. 245. f 



