3i8 TERTIARY VOLCANO OF MULL. 



remain resting upon the older acidic agglomerates, and penetrated by 

 dykes of gabbro. When gabbro is poured out as lava it becomes 

 basalt and dolerite ; and floods of basalt extend round Ben More in 

 unbroken sheets. They cover the whole of the North of Mull, and 

 lap along the west coast of Morvern. Indications of their extent are 

 seen in the now isolated patches which form Ulva, Staffa, the Treshnish 

 Islands, and the rock covering the Ross of Mull, but the lava extended 

 much beyond its present limits. Ben Yattan shows that it flowed far 

 into Morvern, while southward it may have extended into the Firth 

 of Lome. The circumference of the base of this volcano could not 

 have been less than 40 miles, and judging from the proportions of 

 Etna, its height may have been 14,000 feet. Like Etna the old Mull 

 volcano appears to have been covered with small cones towards the 

 close of the second period of its activity. One of these minor cones, 

 or puys, called Ben Sarsta, rises to a height of between 800 or 900 

 feet. It is situate behind Tobermory, and consists of a central core of 

 gabbro, surrounded by basalt or dolerite which is altered by contact 

 with the central intrusive mass. This gabbro may well have been 

 the site of the old crater, and in its centre is a small lake. The 

 country round is so covered with bog that neither the lavas nor ashes 

 poured out from this little cone can be distinguished, even if they 

 escaped the denudation under which the great volcano suffered. The 

 central mountain mass of Mull as it is now left has a diameter of 12 

 miles, and consists of peaks which rise to a height of 2000 to 3000 

 feet, the highest being Ben More. 



Staffa. The cave of Staffa is 

 excavated in vertical prisms of 

 basalt, between rows of which 

 the eye rests on the distant 

 view of lona. Over the cave 

 the basalt is in smaller prisms, 

 lying obliquely. 



Ardnamurchan. Another 

 great volcano existed in the penin- 

 sula of Ardnamurchan. The evi- 

 dences of its activity are not so 

 clear as those seen in Mull. The 



Fig. 66.-Fingars Cave (Staffa). whole of ^ g w Qf ^ penin _ 



sula consists of varieties of gabbro which form wild and barren moun- 

 tains. On the east the gabbro graduates into dolerite. These rocks 

 are similar to those of Mull, and similarly rest upon and break through 

 a series of acidic lavas. The peak of Meal-nan-Con and the neigh- 

 bouring heights to the S.W. are formed by intrusive felsite, which, 

 like that of Mull, passes into granite. It is penetrated on the east 

 and south by sheets and veins of newer amygdaloidal felstone, which 

 are interstratified with beds of ashes and scoriae, full of fragments of 

 the primary and secondary rocks, through which the volcano of 

 Ardnamurchan pierced. 



Here, as in Mull, subordinate volcanic cones of later date appear 



