2 
-_ ee 
Peers, Srcr- i. §2.] SILURIAN, 673 
treme north of Scotland on the same parallel as the Canadian,—that 
species of Maclurea and Raphistoma, resembling those of the St. Lawrence 
_ basin, and Orthocerata bearing large siphuncles, like those of North 
America, Scandinavia, and Russia, should occur in Scotland, and yet be 
scarcely known further south, is at least suggestive of a geographical 
distribution—perhaps even of climatal conditions—not very unlike that 
of more modern times.” From this paleontological decision it follows 
that the overlying conformable schistose series of the Scottish Highlands 
is a mass of metamorphosed Silurian strata. 
In the south-east of Iveland, grey, greenish, and purple grits, and 
grey and dark shales he unconformably upon the Cambrian rocks, and 
contain a few fossils of Landeilo age. They present interstratified beds 
of tuff and felsitic lavas indicating contemporaneous volcanic action. In 
the north-east of the island a broad belt of Lower Silurian rocks runs from 
the coast of Down into the heart of Roscommon and Longford. This 
belt is evidently a prolongation of that in the southern uplands of 
Seotland. It is marked by the occurrence of similar dark anthracitic 
shales crowded with graptolites. The richest fossiliferous localities 
among the Jrish Lower Silurian rocks are found at the Chair of Kildare, 
Portrane near Dublin, Pomeroy in Tyrone, and Lisbellan in Fermanagh, 
‘where small protrusions of the older rocks rise as oases among the 
surrounding later formations. Portlock brought the northern and 
western localities to light, and Murchison pointed out that, while a 
number of the trilobites (Trinucleus, Phacops, Calymene, Illznus), as well 
as the simple plated Orthidx, Leptzeenz, and Strophomene, some spiral 
shells, and many Orthocerata, are specifically identical with those from 
the typical Caradoc and Bala beds of Shropshire and Wales, yet they 
are associated with peculiar forms, first discovered in Ireland, and very 
rare elsewhere in the British Islands. Among these distinctive fossils he 
cites the trilobites, Remopleurides, Harpes, Amphion, and Bronteus, with 
smooth forms of Asaphus (Isotelus), which, though abundant in Ireland and 
America, seldom occur in Wales or England, and never on the Continent.? 
_ In the north and west of Ireland a large area of surface is occupied 
by érystalline rocks—gneiss, schists, quartz-rocks, limestone, granite, 
&¢c.—which are manifestly a continuation of those of the Highlands of 
Scotland. They run south-westward parallel with the belt of unaltered 
Lower Silurian rocks from which, in some places, as in county Tyrone, 
they are only a few miles distant. The district of Pomeroy, so rich in 
Silurian fossils, promises to afford the greatest light on the interesting 
but difficult problem of the metamorphism of the Lower Silurian rocks of 
the Scottish Highlands and the north-west of Ireland. It will be seen 
from the evidence furnished by the sections in West Mayo (p. 685) that 
the metamorphism must have taken place prior to the deposition of the 
Upper Silurian rocks of the west of Ireland. 
Upper Silurian.—The series of rocks in the British Islands classed 
as Upper Silurian occurs in two very distinct types. So great indeed 
is the contrast between these types that it is only by a comparison of 
organic remains that the whole has been grouped together as the deposits 
of one great geological period. In the original region described by Mur- 
-chison, and from which his type of the system was taken, the strata are 
1 Quart. Journ. Geol. Soc, xx. 381. 
2 « Siluria,” p. 174. 
2x 

