Chap. VIII.] CAMBEIAN AND LOWEK SILTJKIAN IN IRELAND. 173 
The earliest publication, however, in which any of the fossils of very 
ancient date were illustrated was the valuable detailed Survey of parts of 
the North of Ireland by the late General Portlock, who, from a small district 
of schistose greywacke, near Pomeroy in Tyrone, described a multitude of 
interesting types, which enabled him distinctly to class these strata with 
the Lower Silurian group *. 
When the first edition of Griffith's great Geological Map of Ireland 
appeared, the existence of rocks containing fossils of this age had been 
ascertained in a few localities only, the term Silurian being restricted by 
him to the precise spots from which those organic remains were obtained ; 
and an important work on the ' Silurian Fossils of Ireland ' was printed 
in 1846, under his auspices. But the progress of research, of late years, 
on the part of Sir Richard Griffith and his assistants, has much extended 
the areas in Ireland in which Silurian fossils occur, and has prodigiously 
increased the lists of organic remains. 
The labours of the Government Surveyors, first under Professor Oldham, 
and now directed by Mr. Jukes, have since led to a still more methodical 
elucidation of these rocks. In this way it has been ascertained that in 
the one term of ' greywacke,' the following masses were grouped : — First, 
the Cambrian or dull-red and purplish schist and grey quartzite, to the 
south of Dublin, the equivalent of the Longmynd, in which no other 
fossils, as before stated, have been found, except Fucoids and two spe- 
cies of Oldhamia (p. 29). Occupying the headlands of Bray, the moun- 
tains of the Sugarloaf, and the gorge of the Dargle, to the south side 
of Dublin Bay (and probably many other parts of the kingdom), these 
older or bottom rocks have recently been determined to be overlapped 
unconformably by all the overlying fossiliferous strata to which the term 
Silurian has been applied. "With this physical severance, so completely in 
contrast to the order of succession in Shropshire and North "Wales (p. 31), 
there is, as might be expected, an absence in Ireland of the band of Lingula- 
slates, which in other countries occupies the intermediate horizon. 
The first recognizable strata which overlie the purple schists with 
Oldhamia are dark slaty strata which at intervals are found to contain 
undoubted Lower Silurian remains. These fossiliferous schists occur in 
large but broken bands in the counties of "Wicklow and Wexford, and 
reappear from beneath the Old Red Sandstone and Carboniferous Lime- 
stone in the cliffs of Waterford to the south of the Bay of that name. 
Flanked as these Lower Silurians are, on their western side, by the 
dominant ridges of eruptive granite that form the Wicklow Mountains, 
they are there either in a crystalline or subcrystalline state, and thus 
exhibit a third condition of the greywacke of old authors, which in the 
new maps of the Geological Surveyors is distinguished by a darker tint. 
The silvery sheets of the well-known waterfall of Powerscourt are pre- 
* See Eeport on Londonderry and parts of Tyrone and Fermanagh. Dublin, 1843. 
