CHAP. XL1 
VENTS OF THE BASALT-PLATEAUX 
289 
a picturesque crag, in front of the precipice of Compass Hill (Fig. 306). 
But the same rock may be traced southward to the Coroglion Mor, and 
north-westward in the lower part of the cliffs to a little beyond the sea-stack 
of An Stoll. It has thus a diameter of at least 3000 feet. Westward it 
passes under the conglomerate described in Chapter xxxviii. Its eastern 
extension has been concealed by the sea. 
The materials that fill this vent consist of a typical agglomerate com- 
posed entirely, or almost entirely, of volcanic detritus. The embedded blocks 
vary up to eight feet in diameter or even more. They are chiefly fragments 
of various basalts and andesites, generally vesicular or amygdaloidal. Some 
VOL. 11 u 
