AyyiVERSART ADDRESS OF THE PRESIDENT. I 75 



in these, formed an important feature in the volcanic operations, 

 cttlminatiug here and there in the opening of vents and the ontflo^v 

 of lava at the surface, with generally but little tuff. Of this second 

 type Tve have only two illustrations in our geological history — that 

 in the Archaean gneiss of the north-west Highlands, and that in the 

 great system of Tertiar}' dykes in Scotland, the north of England, 

 and the north of Ireland. In the Archaean area no trace remains 

 of any superficial lavas, though the parallel dykes are so abundant ; 

 in the Tertiary series large sheets of the erupted lavas still remain 

 in Antrim and the Inner Hebrides. It is curious that the first and 

 the last igneous operations in this country should have been in some 

 respects so much alike and so different from those which intervened 

 between them. 



9. The sequence of events from the beginning to the end of a 

 volcanic period appears to have generally followed a well-defined 

 order. In the case of the fissure-eruptions, vents seem to have 

 been opened along the lines of some of the numerous dykes. In 

 other cases, the communications with the surface were effected by 

 successive explosions which finally blew out an orifice at the surface 

 with no visible relation to any fissures or dykes. Of course beneath 

 the formations that now form the surface, and through which the 

 necks rise, there may be lines of fault or weakness in older rocks 

 which we cannot see. But, in what can be actually examined, the 

 vents have commonly been drilled through rocks independently of 

 faults. 



The discharge of explosive vapours was sometimes the first and 

 only effort of volcanic energy. Generally, however, fragmentary 

 volcanic materials were ejected, or, if the eruption was more vigorous, 

 lava was poured out. In a vast number of cases, especially in the 

 later ages of Palaeozoic time, only ashes were projected, and cones of 

 tuff were formed. In the earlier ages, on the other hand, there was 

 a much larger proportion of lava expelled. Towards the close of a 

 volcanic period the vents were gradually choked up with the frag- 

 mentary materials that were ejected from and fell back into them. 

 Occasionally, during the process of extinction, an explosion might, 

 still occur and clear the chimney, so as to allow of the uprise of a 

 column of molten rock which solidified there ; or the sides of the 

 crater as well as of the cavernous funnel underneath fell in and 

 filled up the passage. Heated vapours sometimes continued to. 

 ascend through the debris in the vent, and to produce on it a marked 

 metamorphism. 



