l68 PROCEEDINGS. ANNIVERSARY ADDRESS. 



The uprise of these enormous domes of acid material is one of 

 the most singular features in the geology of the Inner Hebrides. It 

 has probably taken place partly through older vents, of which there 

 are here and there some remains. Considerable alteration of the 

 other volcanic rocks may be traced around the bosses, while the 

 Cambrian (Lower Silurian) limestone through which they rise is 

 changed into a white crystalline marble. 



In Antrim the bedded basalts have been invaded by trachyte, but 

 a band of tuffs intercalated between the upper and lower group of 

 basalts consists mainly of trachyte debris, proving that trachytic 

 rocks already existed at the surface, or were ejected in a fragmentary 

 form before the later half of the basalts was poured out. In the 

 Inner Hebrides, however, no evidence has yet been found that any 

 of the granophyric ])uys established a connexion with the surface 

 and generated superficial outflows of acid lavas. 



One solitary instance is known of the discharge of an acid lava 

 at the surface of the ground — that of the Scuir of Eigg. In this 

 case we see that the basalt-plateau had been enormously denuded 

 before the pitchstone of that famous hill was poured out into the 

 eroded stream-channel in which it cooled and solidified, A long 

 interval of time, therefore, elapsed between the cessation of the 

 basalt streams of Eigg and the outflow of the pitchstone. 



It is interesting to notice further that at least some, if not all, of 

 the numerous basalt dykes of the Eigg plateau are older than the 

 stream-channel, for they are cut off by the shingle of that channel. 

 The uprise of these dykes consequently took place long before the 

 outflow of the pitchstone. In this locality, therefore, the acid rock 

 is the latest volcanic protrusion. Among the great granophyre 

 bosses of Skj-e, on the other hand, though it can be proved that 

 they are younger than a vast number of the basalt and andesite 

 dvkes, it is equally certain that a small number of dykes runs 

 across them. In these cases the dykes are the most recent of the 

 erupted masses. Yet, looking at the Tertiary sequence as a whole, 

 we see that it illustrates the general law that the more basic 

 eruptions preceded the more acid. 



With the view of showing at a glance the stratigraphical position 

 and leading petrographical characters of the volcanic zones in the 

 geological history of the British Isles, as detailed in the present 

 Address, the following generalized table is inserted.^ 



^ Mr. B. N. Peach and Mr. J. Home have recently found a band of tuff 

 in the Torridon sandstone of Sutherland. The * Torridonian ' is therefore 

 inserted in the table. 



I 



