264 [Proc. B. N. F. C, 



be adequate to produce such vast level and continuous sheets. 

 They are termed " massive eruptions," and have been held by 

 many " to represent the grand fundamental character of vulca- 

 nism, modern volcanic cones being regarded as merely parasitic 

 excrescences on the subterranean lava reservoirs, very much in 

 the relation of minor cinder cones to their parent volcanoes."* 

 A distinguishing feature of lavas erupted in this way is their 

 horizontal bedding and the rarity of the dykes which intersect 

 them, relatively to local outbursts such as have formed the 

 Islands of Madeira or Teneriffe. These contrast strongly with 

 them, being riddled in every direction by veins and dykes, 

 while in Iceland I rode for a long day down a valley between 

 walls of basalt some 2000 feet in height, and saw only one dyke. 

 In Antrim, which is away from the centre, the dykes are 

 apparently more numerous. Again, while Professor Hull and 

 others in Ireland, and Professors Geikie and Judd in Scotland, 

 mention the occurrence of volcanic necks, or true craters. Dr. 

 James Geikie met no trace of such in the Faroes, nor did I 

 among the Tertiary basalts of Iceland. I often saw knolls or 

 bosses projecting above the general level of the basalts, but 

 these always proved to be the last remaining fragments of layers 

 that had otherwise been entirely denuded, and it struck me 

 that if overgrown, these would have been mistaken for necks. 



These formations were not erupted at once. In Ireland, 

 they have been grouped by Professor Hull into three classes, 

 the oldest of which includes highly silicated felspathic trachytes, 

 porphyry, pearlstone, and pitchstone. The second comprises 

 basic beds of amygdaloid with bands of bole, volcanic ash, etc. 

 The third is formed of massive sheets of columnar basalt. A 

 very long interval, marked by the changed characters of the 

 lavas, is supposed to have intervened between each stage, and 

 lesser intervals are marked by the layers of bole, which so fre- 

 quently intervene between the lavas. Each layer marks, how- 

 ever, a period sufficiently long for a considerable depth of the 

 solid basalt to have decomposed into vegetable mould, and 



* Geikie, Text Book of Geology, 1882, p. 256. 



