THE TERTIARY VOLCANOES 
BOOK VIII 
388 
the eastern side of the Blaven group of hills. Mr. Harker has traced a great 
development of granophyre on the west side of these hills, where the acid 
rock sends apophyses both into the bedded basalts and into the gabbros. 
Combining the results of observations made not only in Skye but in Mull, 
Bum and Ardnamurchan, I shall here give a fuller account of the meta- 
morphism of the basalts, to which frequent allusion has been made as one of 
the evidences of the posteriority of the eruptive bosses of rock round which 
it occurs . 1 The field-geologist observes that the basalts, as they are traced 
towards these bosses, lose their usual external characters. They no longer 
weather into spheroidal blocks with a rich brown loam, but project in much 
jointed crags, and their hard rugged surface shows when broken a thin white 
crust, beneath which the rock appears black or dark bluish-grey, dull and 
splintery. They are generally veined with minute threads or strings of 
Fio. 349. — Section at north end of Beinn 11 a Cro, Skye. 
a, basalt, dolerite and gabbro ; i, granophyre of Beinn na Cro ; 6 1 , dyke of granophyre ; c c, basalt dykes. 
calcite, epidote and quartz, which form a yellowish-brown network that 
projects above the rest of the weathered surface. TV here they are amygdal- 
oidal, the kernels no longer decay away or drop out, leaving the empty 
smooth-surfaced cells, but remain as if they graduated into the surrounding 
1 Many years ago 1 was much struck with the evidence of alteration in the igneous rooks of 
Mull, and referred to it in several papers, Proc. Pay. Sue. Edit 1 . (1866-67) vol. vi. p. 73 ; Quart. 
Journ. Gaol. Sue. xxvii. (1871) p. 282, note. The subject was more fully discussed in my memoir 
in the Trans. Roy. Soc. Edin. vol. xxxv. (1888) p. 167, from which the account in the text is 
taken. Prof. Judd has more recently referred the alteration to solfataric action (Quart. Journ. 
Geol. Soc. xlvi. 1890, p. 341). As already mentioned, I have been unable to detect evidence of such 
action. The alteration is always intimately connected with the presence of intrusive masses, and 
it affects indifferently any part of the basalt-plateaux which may chance to lie next to these 
masses. The bedded lavas can be traced stop by step from their usual unaltered condition in the 
plateaux to their metamorphosed state next to the eruptive rocks. The nature or degree of the 
metamorphism has doubtless somewhat varied with the composition and structure of the rocks 
affected, and with the character and mass of the eruptive material ; but it is certainly not con- 
fined to the older parts of the plateaux, nor to any supposed pre-basaltic group of andesites. I 
have found no evidence that such a group anywhere preceded the plateau-basalts. The ande- 
sites, so far at least as my observations go, were erupted at intervals during the plateau period, 
and alternate with the true basalts. The greatest accumulation of them lies not below but above 
the general body of the basalts, in the “pale group” of Mull. Nor even if the term “propylite 
be adopted for these altered rocks, can it be applied to any special horizon in the volcanic series. 
The alteration of the basic rocks by the granophyre of St. Kilda will be described in the account 
of that island in Chapter xlvii. 
