Vol. 52.] BASALT-PLATEAUX OF NORTH-WESTERN EUROPE. 395 



between the granophyre and the shales. There can be little doubt 

 that this intrusion is later than the granophyre. We have here a 

 basic sill interposed at the bottom of the acid sheet ; but in this 

 case we can connect the sill with the actual fissure up which its 

 molten material was impelled. 



Fig. 30. — Section of granopliyre-sill resting on Lower Lias shales, 

 with a dyke of basalt passing laterally into a sill. 



Some remarkable illustrations of the threefold arrangement of 

 compound sills have recently been observed and mapped by Mr. Harkcr 

 in the Broadford district, one of great interest occurring on the 

 shore at Irishman's Point in Broadford Bay. 



"VIII. Modern Volcanic Action in Iceland, as illustrative of the 

 History of the Basalt-plateaux of North-western Europe. 



Beyond the Faroe Islands, at the further end of the WyvilJe- 

 Thomson ridge, which stretches across the bottom of that part of 

 the Atlantic Ocean, another basalt-plateau rises in Iceland, pre- 

 senting many of the familiar characters of those described in this 

 paper, and probably belonging to the same geological period. The 

 bottom of these Icelandic Tertiary basalts is everywhere concealed 

 under the sea. Yet their visible portion shows them to be probably 

 more than 3000 metres in thickness. 



An especial interest belongs to this Icelandic plateau because 

 volcanic action is still vigorous upon it at the present day. A long 

 series of eruptions has taken place there since the Glacial Period. 

 There were likewise abundant pre-glacial eruptions. So far indeed as 

 we know, there is no evidence of any important cessation of the 

 subterranean activity since Tertiary time. 1 The existing volcanic 

 phenomena may with probability be regarded as the survival of 

 those which were so widely manifested over the Icelandic area and 

 the North-west of Europe in tho older Tertiary ages. A careful 

 study of them may therefore be expected to throw light on the 

 history of the Tertiary basaltic plateaux; while, on the other hand, 

 the thorough dissection of these plateaux by the denuding agencies 

 1 See Johnstou-Lavis, Scottish Geogr. Mag. 1895, p. 442. 



