CHAP. XLI 
VENTS OF THE BASALT-PLATEAUX 
275 
under a high power, the interstitial matter is seen to consist mainly of a 
greenish-brown isotropic substance, in which are inclosed small crystals ol 
augite, skeleton-forms and microlites of felspar, sometimes in stellate aggie- 
gates, as well as club-shaped, cruciform, arrow-headed and often crested 
microlites of magnetite. 
Towering prominently above the flat basalt sheets, this neck has an oval 
form, measuring about half a mile in length by a quarter ot a mile in 
breadth. Its central portion, however, instead of rising into a rugged hill- 
top, as is usually the case, sinks into a deep hollow, which is filled with 
water, and reminds one of a true crater-lake (Figs. 299, 300). The 
middle of the neck is thus concealed from view, and we can only examine 
Fig. 300. Interior of the Volcanic Neck of ’S Airile Beinne, near Tobermory, Mull. 
the hard prominent ring of dolerite that surrounds the tain. The 
material occupying the hollow may be softer than that ot the ring, and 
may have been scooped out by denudation. What we now see may not be 
the original surface, but may have been exposed after the removal of possibly 
hundreds of feet of overlying material. On the other hand, it is conceivable 
that the hollow is really a crater-lake which was filled up with detritus 
and may have been overspread with basalt, since removed. It may be 
suggestively compared with the. crater-hollows revealed bj denudation on 
the cliffs of Stromb and Portree Harbour, which will be described in a latei 
part of this chapter. Possibly some more easily removable agglomerate, 
representing an eruption later than that of the dolerite, may occupy the 
centre of the volcanic pipe. 
One of the most interesting features of this vent is to be found in its 
relation to the surrounding basalts. The marginal parts of the rock along 
