CHAP. XLVII THE ACID ROCKS OF NORTH-EAST IRELAND 
421 
underlie Upper Silurian grits and shales. So far as regards the contact of 
these rocks with the disrupted sedimentary formations, all that can be 
asserted is that the granite must he later than at least the older part of the 
Upper Silurian period. But for at least two reasons, tire eruptive rocks may be 
regarded with some confidence as part of the Tertiary series. In the first 
place, there is a strong petrographical resemblance between the Mourne 
Mountain granite and that of the Island of Arran and the granitic parts of 
the granophyre of the Western Isles. And this resemblance is so close as to 
furnish a cogent argument in favour of grouping all these rocks together as 
parts of one geologically contemporaneous series. I 11 the second place, the 
Mourne Mountain granite abruptly cuts off a large number of basic dykes 
which, running in a general X.N.W. direction, may be looked upon as almost 
certainly members of the Tertiary system of protrusions. 
The manner in which the granite of the district behaves towards 
certain detached areas of Silurian strata with their accompanying dykes is 
one of the most astonishing features in the whole assemblage of intrusive 
rocks in Britain. As has been excellently shown in the Geological Survey 
Map and sections by Mr. W. A. Traill, the granite has carried up on its 
surface broad cakes of vertical Silurian strata, together with all their network 
of dykes . 1 A cake of this kind, from 50 to about 200 feet thick and nearly 
two miles broad, has been bodily uplifted from the rest of the mass and 
carried upward by the granite, so that the truncated ends of the beds of 
grit and shale with their system of dykes stand upon a platform of granite, 
from which also numerous veins penetrate them. There can be little doubt 
that the basic dykes thus broken through are parts of the great Tertiary 
system, and if so, the granite which disrupts them cannot be older than 
Tertiary time. 
Besides the older basic dykes disrupted by the granite, a younger but 
much less abundant series traverses that rock, and also follows a general 
north-westerly direction. These later dykes in some cases cross more acid 
dykes which have risen through the granite. There is no trace of any 
superficial discharge from the Mourne Mountain area. But from the analogy 
of other districts we may easily conceive that the granite represents the 
underground parts of volcanic material which has now been entirely 
removed. 
b. Slieve Foye and Barnavave District . — This area embraces the moun- 
tainous ground lying between Carlingford Lough and Dundalk Bay, and 
culminating in Slieve Foye (1935 feet). It measures roughly about six 
miles in extreme length and four miles in breadth. 
The remarkable assemblage of basic and acid materials in this area has 
received considerable attention from geologists. The relative order of the 
1 See Sheets 60, 61 and 71 of the one-inch map of the Geological Survey of Ireland, and 
Sheets 22, 23 and 24 of the Horizontal Sections. The Explanation to these Sheets of the map 
was written by Professor Hull, Mr. Traill having previously retired from the service. The 
Mourne Mountain area is now undergoing critical revision by Prof. Sollas for the Geological 
Survey, and important additional material for the elucidation of this district may be expected 
from liim. 
