DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



139 



of these can be distinctly seen breaking across or ending off between the bedded basalts 

 which here dip gently into the hill (fig. 41). A conspicuous band of coarse basalt- 

 agglomerate, containing blocks of compact and amygdaloidal basalt a yard or more in 

 diameter, shows by the excessive induration of its dull-green matrix the general alteration 

 which the rocks of the basalt-plateau have here undergone. An almost incredible 

 number of veins of fine basalt, porphyry, and felsite has been injected into these rocks — 

 a structure which is precisely a counterpart of what occurs under the main body of gabbro 

 in Skye, Ardnamurchan, and Rum. 



The gabbro mass of the Ben Buy ridge is thus undoubtedly a huge overlying sheet, 

 which probably reaches a thickness of at least 800 feet. It seems to descend rather 

 across the bedding into the hollow of Glen More, and possibly its main pipe of supply 

 lay in that direction. Being enormously thicker than any other sheet in the island, it 

 exhibits the crystalline peculiarities which are so well developed in the central portions 

 of the larger bosses of gabbro. It presents more coarsely crystalline varieties than appear 

 in the thinner sheets, some portions showing crystals of diallage and felspar upwards of 



Fig. 41.— Altered Plateau-Basalts invaded by Gabbro, and with a Dyke of prismatic Basalt cutting both rocks, North 

 Slope of Ben Buy, Mull, aa, amygdaloidal basalt, much altered; b, gabbro; c, finely prismatic basalt. 



an inch in length. It likewise contains admirable examples of segregation-structure, 

 which, as in Skye and elsewhere, is best developed where the texture becomes especially 

 coarse. These veins or seams, in which the constituent minerals have crystallised out in 

 more definite and conspicuous forms, here and there succeed each other so quickly as to 

 impart a kind of bedded or foliated look to the body of rock in which they occur, recalling 

 the aspect of some coarsely crystalline granitoid gneiss. Occasionally, on the exposed 

 faces of crags, portions of such veins are seen to be detached and enveloped in the finer 

 surrounding matrix. This pseudo-stratification is to be distinguished from another struc- 

 ture in which thick belts or bands of coarser and finer texture alternate, and give an 

 appearance of bedding to the mass. These bands run generally parallel with beds of 

 highly indurated basalt, which appear to be separated portions of the ordinary rocks of 

 the plateau. They may be taken to indicate that the thick sheet of Ben Buy is not 

 the result of one but of many uprises of gabbro. 



Of the thinner sheets of dolerite and gabbro little need here be said. I have referred 

 to their great abundance in the range of eastern hills that rise from the Sound of Mull 



