172 
SILUEIA, 
[Chap. VIII. 
. S be d d 
- ^ m © 
O es too Ph 
discover that the quartz-rocks, lime- 
stones, and great overlying gneissose 
masses of the north-west undulate 
in vast folds or waves across the coun- 
try, until, along the Highland fron- 
tier, they pass under the Lower Old 
Red Sandstone. The details of this 
structural arrangement were sketched 
hy my colleague and myself in the 
paper already referred to, and are 
further illustrated in the sections 
given along the margin of our Geo- 
logical Map of Scotland. From these 
sections,as well as from that which he 
has now prepared, it will be seen that, 
just as the metamorphosed gneissose 
and slaty rocks pass under the Old Red 
Sandstone along the northern margin 
of the broad central lowlands, so 
from its southern margin, unmeta- 
morphosed grits, greywackes, and 
shales rise from under the Coal-fields 
and Old Red Sandstone, and undu- 
late in an unbroken series of anticlinal 
and synclinal curves up to the English 
Border. Hence the metamorphic 
rocks of the Highlands are, in all 
likelihood, the geological equivalents 
of the unaltered Silurian rocks of 
the Southern Uplands. 
Silurian Bocks of Ireland. — Rocks 
which, from their included organic 
remains, must be classed as Silurian 
occupy considerable portions of Ire- 
land, whilst certain stratified crys- 
talline masses of this kingdom are 
also probably (as in the Highlands 
of Scotland) of the same age. 
In a retrospect of the progressive 
steps in the classification of Irish 
Palaeozoic rocks, allusion is first due 
to the veteran geologist Weaver*. 
* See Trans. G-eol. Soc. Lond. vol. v. p. 117; 
2nd ser. vol. v. p. 1 et seq. 
