CHAP. XXXV 
ORIGIN AND HISTORY OF THE DYKES 
175 
the same is certainly true ot the augite which occasionally accompanies them 
and of the quartz that appears in some examples. 1 
In the Carlingford district of the North-east of Ireland, similar evidence 
has been obtained that one series of dykes preceded and another followed 
the protrusion ol the granites and granophyre which are in all probability 
geologically coeval with the acid bosses of the Inner Hebrides. The 
distinction was observed and mapped by Mr. Traill for the Geological 
Survey. Professor Sollas in recently confirming these observations has not 
noticed any striking difference between the pre-granite and post-granite 
dykes, the whole appearing to consist of the same coarsely porphyritic 
material. 2 
While the eruption of the granophyre bosses furnishes proof that the 
dykes are not all of the same age, other evidence may be gathered to show 
how much older some of the dykes are than the youngest lava-streams in 
the volcanic history of Tertiary time in Britain. The Scuir of Eigg, to 
which fuller reference will be made in Chapter xxxviii., is formed of a 
mass of pitchstone, which has filled up an ancient valley eroded out of 
the terraced basalts of the plateaux. At both ends of the ridge, these basalts 
are seen to be traversed by dykes that are abruptly cut off by the shingle 
ol the old river-bed which the pitchstone has occupied (Figs. 279, 282) 
It is thus evident that, though these dykes are younger than the plateau- 
basalts, they are much older than the excavation of the valley out of these 
basalts, and still older than the eruption of pitchstone. The latter rock 
probably belongs to the close of the period of lava-eruptions. The enormous 
denudation of the basalt-plateaux after the injection of the dykes and before 
the outflow of the pitchstone affords a convincing proof of the vastness of 
the interval between the eruption of the two kinds of rock. 3 
It is thus demonstrable that the dykes which in Britain form part of 
the great Tertiary volcanic series, were not all produced at one epoch, but 
belong, to at least two (and probably to many more) episodes in one long 
volcanic history. As they rise through every member of that series 
of rocks (save the pitchstones), some of them must be among the latest 
records of the prolonged volcanic activity. But, on the other hand, some 
probably go back to the very beginning of the Tertiary volcanic period. 
20. ORIGIN AND HISTORY OF THE DYKES 
Preference has already been made to the doubt expressed by Macculloch 
whether the dykes in Skye had been filled in from above or from below. 
That the dykes of the country as a whole were supplied from above, was 
the view entertained and enforced by Boue. He introduces the subject with 
Annual Report of the Director-General of the Geological Survey in Report of Science and Art 
Department for 1895. 
- See Sheets 59, 60, and 71 of the Geological Survey Map of Ireland ; Professor Sollas, Trans. 
102 / Insh Acad. vol. xxx. (1894), p. 477 ; and Annual Report of the Director-General of the 
Geological Survey for 1895. 
Quart. lour. Gcol. Soc. xiv. p. 1. 
