DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 109 



mountain are so indurated and shattered that it is difficult to separate them from each 

 other and determine their relative positions. But, so far as I could ascertain, these 

 basalts are fragments of beds that overlie the agglomerate (fig. 31). This is not the only- 

 place along the flanks of the Ked Hills where portions of the 

 bedded basalts have survived. Other localities will be sub- 

 sequently alluded to. 



The great vent of Strath has been drilled through the 

 Lower Silurian limestone, and as the result of protracted 

 denudation it now towers steeply five or six hundred feet 

 above that formation on the floor of the valley. Of the 

 material discharged_ from it over the surrounding country no Fig- 31.— Diagram to show the 



TTT . „ „ , „ probable relations of the rocks 



certain trace now remains. We may inter irom the nature 01 n the southern flank of Beinn 

 the rock which fills it that towards the end, if not from the Dear g Bhea s- «> agglomerate ; 



, .. . . b, amygdaloidal and compact 



beginning ot its activity, its discharges consisted mainly 01 basalt-rocks ; c, granophyre. 

 dust and stones. A crater, of which the pipe was two miles in 



diameter, must surely have sent its fragmentary materials far and wide over the sur- 

 rounding region. But on the bare platform of older rocks to the south not a vestige 

 of these erupted materials can now be found. Westward the escarpment of Strathaird 

 remains to assure us that no thick showers of ashes fell at even so short a distance as two 

 miles, either before or during the outpouring of the successive basalt sheets still remaining 

 there. We may therefore conclude with some confidence that here, as at Ardnamurchan, 

 the vent must be younger than at least the older parts of the basalt-plateau. Unfortun- 

 ately the uprise of the large bosses of granophyre that stretch from the Eed Hills to Loch 

 Sligachan has entirely destroyed the vent and its connections in that direction. There 

 is no certain proof that any molten rock ever issued from this orifice, unless we suppose 

 the fragmentary patches of amygdaloid on the southern flank of Beinn Dearg Bheag to 

 be portions of flows that proceeded from this centre of eruption. I have little doubt that 

 the basalt-plateau which still remains in Strathaird formerly extended eastwards over 

 Strath and northwards across the site of the Red Hills and Cuillins, joining on to 

 the continuous tableland north of Lochs Brittle and Sligachan. How much of the 

 plateau had been built up here before the outburst of the vent can hardly be conjectured. 

 The agglomerate may possibly, of course, belong to the very latest period of the plateau- 

 eruptions, or even to a later phase of Tertia^ volcanic history. The impression, however, 

 made on my mind by a careful study of the evidence from this and the other districts is 

 that the necks of agglomerate, like those of dolerite and basalt, really belong to different 

 epochs of the plateau period ; that they mark for us some of the vents from which the 

 materials of the plateaux were emitted. 



The example of Car rick- a-raide is peculiarly suggestive when we regard it in 

 connection with the great Strath vent. Already the progress of denudation has removed 

 at least half of the layer of dust and stones which, thrown out from that little orifice, fell 

 over the bare chalk-wolds and black basalt-fields of Antrim. The neck that marks the 



VOL. XXXV. PART 2. P 



