46 SECTIONAL ADDRESSES. 



crust where the temperature gradients would permit the cooling of the 

 magma below the crystallization point of some or all of its constituents. 

 The origin of localised centres of highly differentiated intrusive rocks, such 

 as those with which we have to deal, appears to me to be inseparable 

 from the idea of pronounced local subsidences. 



The first expression of local activity, as in Skye, Mull and Ardnamurchan, 

 usually takes the form of vent-agglomerates, or explosion-breccias, which 

 are the result of the explosive shattering and brecciation of the country- 

 rock — most frequently the plateau lavas. The magma responsible for the 

 formation of these agglomerates is invariably of acid composition, and 

 it may have actually broken through, producing a true vent, or merely 

 effected intense shattering of its roof. These agglomerate-masses or vents 

 were presumably formed by the action of the highest liquid portion of a 

 more or less completely differentiated magma that filled a local reservoir. 

 If the magma prior to differentiation had ascended to within a reasonable 

 distance from the surface, as differentiation progressed the pressure due 

 to accumulation of gases in the acid differentiate would increase, and cause 

 it to break through and shatter the remaining portion of the crust. The 

 first intimation of its presence would be the formation of agglomerates, 

 followed or accompanied by the outpouring of acid igneous material of 

 rhyolitic or trachytic composition. Such appears to have been the manner 

 of formation of the Kilchrist vent in Skye, which breaks abruptly through 

 the adjacent strata and has almost vertical sides with a roughly cylin- 

 drical form. Vent-breccia or agglomerate plays a very significant and 

 conspicuous part in the geology of central Mull, and its distribution 

 clearly outlines the two important calderas of that region. Although 

 explosive action on the part of an acid magma was repeated at several 

 periods in the history of Mull, such was one of the earliest, if not the 

 earliest, manifestations of central activity. In Ardnamurchan, in the 

 neighbourhood of Ben Hiant, and conforming to the outline of the 

 plutonic centre, large accumulations of vent-agglomerate break through 

 the older strata and are associated with rocks of trachytic character and 

 composition. 



It appears to be fairly certain that the intrusive centres of Skye, Mull, 

 Ardnamurchan and probably central Arran were marked out by the 

 explosive breaking through of a magma of acid composition. In Mull 

 there is evidence of the strongest kind that magma continued, under 

 repeated subsidence, to be erupted with the temporary establishment of 

 a volcano of Hawaiian type. 



From this point onwards in each centre we enter a period during which 

 all the major plutonic masses were intruded, and I should now like to 

 offer some generalizations on the forms they have adopted. A glance at 

 the published geological maps of Skye and more particularly of Mull will 

 at once reveal the fact that the major intrusions have circular, annular, 

 or crescentic form around some central point. A review of all the Tertiary 

 plutonic masses of Britain, as well as those of south-east Iceland, brings 

 out the fact that laccolitic intrusions are the exception. By laccolitic I 

 mean intrusions that have definitely forced up and displaced pre-existing 

 rocks to make room for themselves, or occupy domed regions, and have a 

 well-defined base. Most of the Tertiary plutonic masses appear to have 



