Vol. 6.2.]. STETJCTCRE OP THE SGTJKK.OF EIGG. 47 



has escaped breaking up, and the pale band is seen abutting upon 

 undisturbed and unaltered rock (see P 1 in fig. 6, p. 60). In 

 addition to the blocks of pitchstone (and felsite), the decomposed 

 matrix encloses in places a few pieces of a blacker and more 

 completely vitreous rock, without conspicuous felspar-crystals, which 

 may be taken to represent a more perfectly glassy selvage to the 

 pitchstone-sheet. There are also fragments of extraneous origin, 

 which are sometimes rather abundant at the base of the pale band, 

 but become rare towards its top. Excepting one locality, to be 

 described below, these extraneous fragments are exclusively of 

 basalt and dolerite, evidently picked up from the subjacent rocks. 

 The inclusion of these is not necessarily connected with the 

 brecciation, for they are found also in the base of the pitchstone 

 where it is unbroken and unaltered : for example, in the spur on 

 the north-eastern side of the ridge near Loch na Mna. Moire. 



The general geological relations of the pitchstones to the basic 

 rocks have at different times received very different interpretations. 

 Macculloch l (1819) does not explicitly discuss the point. He 

 remarks that the rock of the Sgurr may in places be 



' seen resting on the same trap with which it alternates, and thus forming .... 

 the uppermost rock of this interesting island.' 



The ' trap ' here referred to is the felsite which is so intimately 

 bound up with the pitchstone itself; but the concluding words seem 

 to imply that pitchstone and felsite together constitute merely the 

 uppermost member of the general sequence. This ignores the 

 obviously transgressive junction, so clearly exhibited at the eastern 

 end of the ridge. 



Von CEynhausen and von Dechen 2 (1829), as appears from their 

 brief reference and the accompanying figure, considered the pitch- 

 stone to be intruded through the basalts, and indeed they liken it to 

 a dyke. Hay Cunningham 3 took the same view, which he illus- 

 trated by a most uncompromising diagram; and James Nicol 4 

 arrived at a like conclusion. It is difficult to understand how this 

 dyke-hypothesis could ever be entertained, in view of the manifestly 

 stratiform disposition of the pitchstone-mass. At numerous places 

 along the base it is possible to penetrate for several yards under the 

 great sheet, and to see its base, like an inverted pavement, overhead. 



Hugh Miller (1858), like Macculloch, realized that the pitchstone 

 definitely overlies the basalts ; and, though his words are more 

 picturesque than precise, he seems to have regarded it as the result 

 of a volcanic outpouring. He speaks of the rock of the Sgurr as 

 having been ' laid down .... in one fiery layer after another \ 3 It 

 is, however, to Sir Archibald Geikie 6 (1865) that we owe a clear 



1 ' Description of the Western Is. of Scotland' vol. i (1819) p. 522. 

 - Karsten's Archiv fur Min. vol. i (1829) pp. 105-14 & pi. iv. 



3 Mem. Wern. Nat. Hist. Soc. vol. viii (1839) pp. 144-63 & pi. v. 



4 ' Guide to the Geology of Scotland' 1844, pp. 232-33. 



5 * Cruise of the Betsey ' 1858, p. 35. 



G ' Scenery of Scotland ' 1st ed. (1865) pp. 278-82 ; Journ. of Travel & Nat. 

 Hist, vol. i (1868) pp. 14-16 ; Quart. Journ. Geol. Soc. vol. xxvii (1871) pp. 279- 

 310 ; and later works. 



