part 1] AXNIVEESART ADDRESS OF THE PRESIDENT. xciii 



sometimes strongly marked. These Tertiary rocks as a Avhole have 

 much in common witli the Lower Carboniferous igneous rocks of 

 the Midland Valley. Various types of Carboniferous basalts and 

 dolerites are closely reproduced, in addition to the very significant 

 mugearites. Tmch^^tes are less prominent, but are found in Skye, 

 Ardnamurchan, and perliaps elsewhere. In considering the widely 

 distributed amygdaloidal basalts, it is to be remembered that the 

 contents of the vesicles are an integral part of the rock. It is, then, 

 of interest to observe that the Tertiary and Carboniferous lavas are 

 the chief source in Britain of zeolites, while the veiy different lavas 

 of the Old Red Sandstone are the home of agates and onyxes. In 

 not a few of the Tertiary rocks primary zeolites, chiefly but not 

 exclusively analcime, make part of the general fabric, and this is 

 true of lavas, sills, and dykes. Another index of an alkaline com- 

 position is the incoming of a pleochroic titaniferous augite. 



In addition to the Tei*tiary crust-movements already noticed, of 

 simple type and having a vast extension, there were within the same 

 i-egion disturbances of a different order, strongl}^ accentuated and 

 strictly localized about distinct centres. With these is closely 

 related another series of igneous rocks, chiefly in the form of 

 jjlutonic masses with a host of minor intrusions as satellites. The 

 principal centres are in the tract, already mentioned, which embraces 

 the Inner Hebrides and the Clyde Isles, and the most remarkable of 

 all is the Mull centre, situated in the middle of that tract. The 

 i-elations here recall those at the Old Red Sandstone centres of the 

 West Highlands, but show a much higher degree of complexity. 

 They have been unmvelled by the labours of the Geological Survey, 

 and the map of the district, with the accompanying memoir, will 

 doubtless add largely to our knowledge of the mechanism of igneous 

 intinision. Pending the publication of that work, I will ])ass over 

 the subject with a few words. 



The special centres were already marked out in the volcanic 

 phase, or even earlier. In Mull there was a cauldron-subsidence, 

 accompanied by outpourings of ' pillow-lavas " and later a girdle of 

 volcanic vents. The great volcanic vent of Kilchrist in Skye and 

 that of Central Arran also belong to an early date, and may have 

 something of the same significance. At subsequent epochs the 

 vertical movement at the special centres, though not always in the 

 same sense, seems to have been in general one of elevation. Where 

 the local movements were strongly emphasized, the plutonic in- 

 trusions are sharply tmnsgressive, and they stand in evident relation 



