DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 59 



of crystalline rock seems to have offered such resistance to the uprise of molten material 

 through it, that when the subterranean energy was not sufficient to rend it open by great 

 fissures, and thus give rise to dykes, the lavas were forced into such irregular cracks as 

 were made partly in the softer rocks underneath and partly in the cake itself, or found 

 escape along pre-existing divisional planes. In Ardnamurchan, round the Cuillin Hills of 

 Skye, and in Rum, the overlying resisting cover now consists mainly of gabbro sheets. In 

 the east of Skye, in Eigg, and in Antrim, it is made up of the thick mass of the plateau- 

 basalts. 



§ 13. Connection op Dykes with Intrusive Sheets. 



Every field-geologist is aware how seldom he ean actually find the vent or pipe up 

 which rose the igneous rock that now forms those massive beds which he denominates 

 intrusive sheets. He might well be pardoned were he to anticipate that, in a 

 district much traversed by dykes, there should be many examples of intrusive sheets 

 and frequent opportunities of tracing their connection with the fissures from which 

 their material might be supposed to have been supplied. But such an expectation 

 is singularly disappointed by an actual examination of the Tertiary volcanic region 

 of Britain. That there are many intrusive sheets belonging to the great volcanic 

 period with which I am now dealing, I shall endeavour to show in the sequel. But 

 it is quite certain that though these sheets have of course each had its subterannean 

 pipe or fissure of supply, they can only in very rare instances be directly traced to 

 the system of dykes. On the other hand, the districts where great single dykes are 

 most conspicuous, are for the most part free from intrusive sheets, except those of 

 much older date, like the Carboniferous Whin Sill of Durham and the diabases of 

 Linlithgowshire. 



Yet a few interesting examples of the relation of dykes to sheets have been noticed. 

 The earliest observed instances were those figured and described by Macculloch in his 

 Western Islands of Scotland. Among them one has been familiar to geologists from 

 having done duty in text-books of the science for more than half a century. I allude to 

 the diagram of " Trap and Sandstone near Suishnish."* In that drawing seven dykes are 

 shown as rising vertically through the horizontal sandstone, and merging into a thick 

 overlying mass of " trap." The author in his explanation leaves it an open question 

 " whether the intruding material has ascended from below and overflowed the strata, or 

 has descended from the mass," though from the language he uses in his text we may 

 infer that he was inclined to regard the overlying body as the source of the veins below 

 it.t 



The section given by Macculloch, however, does not quite accurately represent the 

 facts. The narrow dykes there drawn have no connection with the overlying sheet, but 

 are part of the abundant series of basaltic dykes found all over Skye. The feeder of the 

 sheet was undoubtedly the thick dyke which descends the steep bank immediately on the 



* Op. cit., pi. xiv. fig. 4. t Vol. i. pp. 384, 385. 



