394 
THE TERTIARY VOLCANOES 
BOOK VIII 
its area cannot but arrest the attention of the observer. Though I was on 
the outlook for dykes in the granophyre, I found only one. Yet immediately 
beyond the eruptive boss they at once appear on either side up to its very 
edge, where they suddenly cease. The conclusion cannot be resisted that 
the protrusion of the acid rock took place after most of the dykes of the 
district had been formed, but before the emission of the very latest dykes, 
which pursue a north-west course across the boss (Fig. 348). 
Some sections on the southern margin of Beinn an Dubhaich complete 
the demonstration that such has been the order of appearance of the rocks. 
Fig. 351. — Section on south side of Beinn an Dubhaich, Skye, showing the truncation of a basalt-dyke 
(6), in Cambrian Limestone (a), by the granite (c) of Beinn an Dubhaich, Skye. 
Near the head of the Allt Leth Slighe (or Half-way Burn), where the 
granite has pushed a long tongue into the limestone, a north-west basalt- 
dyke is abruptly cut off by the main body of the boss and by the protruded 
vein (Fig. 351). Besides this truncation, the acid rock sends out strings 
and threads of its own substance into and across the. dyke, these injected 
portions being as usual of an exceedingly fine felsitic texture. 
Similar evidence may be gathered from the area of the great grano- 
phyre cones further north. The profusion of basalt-dykes in the surround- 
ing rocks stops short at the margin of that area. The comparatively few 
dykes which cross the boundary pursue a general north-west course through 
the granophyre, and, as already remarked, from their dark colour, greater 
durability and straightness of direction, stand out as prominent ribs on the 
flanks of the pale cones which they traverse. 
