ANNIVEESAET ADDEESS OF TSE PRESIDENT. S^ 



loaded slopes of a modern viscous lava, like some of those in the 

 Atrio del Cavallo on Yesuvius. There cannot, therefore, be any 

 doubt that the sandstone so irregularly dispersed through these 

 lavas was introduced originally as loose sand washed in from above 

 so as to fill the numerous rents and cavernous interspaces of the 

 porphyrites. A more striking proof of the subaqueous character of 

 the eruptions could hardly be conceived. 



The general character of the tuffs in the structure of the ground 

 agrees with what might have been expected would mark the accom- 

 paniments of extremely slaggy and viscid lavas. They are in most 

 of the volcanic districts comparatively insignificant in amount, by 

 far the largest proportion of solid material ejected from the various 

 vents having everywhere, save at the Pentland Hills, consisted of 

 streams of lava. Round or within some of the vents the frag- 

 mentary materials attain a remarkable coarseness, as may be seen 

 in the great agglomerates of Dumyat, near Stirling, the largest of 

 which is more than 700 feet thick. These massive accumulations 

 doubtless represent a long series of explosive discharges from the 

 summit of the lava column in one or more adjacent vents. Traced 

 away from the orifices of emission, the tuffs rapidly grow finer in 

 grain, less in thickness, and more mixed with ordinary detritus, 

 until they pass into the non-volcanic sediment or die out between 

 the lava-sheets. 



Good sections, showing the nature and arrangement of the thin 

 intercalations of porphyrite-tuff between the successive outpourings 

 of lava, may be examined on the coast. Thus, near Turnberry 

 Point, in Ayrshire, upwards of a dozen successive flows of lava, 

 with their sandy and ashy intervening layers, are exposed in places 

 upon the beach, and partly also in section along the cliffs on which 

 the ruins of the historic castle of Turnberry stand. Again, along 

 the coast of Forfarshire, from the Eed Head to Montrose, the 

 numerous sheets of porphyrite are separated by layers of dull 

 purplish tuff passing into conglomerate, with blocks of porphyrite 

 a yard or more in diameter. 



But by far the most remarkable tuffs in the whole basin are those 

 felsitic varieties to which I have already referred as occurring in 

 the Pentland Hills. Proceeding from the great vent of the Braid 

 Hills, these tuffs extend south-westwards for 8 or 9 miles, and 

 their peculiar materials, mixed with ordinary sediment, may be 

 traced several miles farther. They occur in successive sheets, which, 

 from a maximum thickness and number at the north end, gradually 



