Chap. VTIL] LOWER SILUKIAN, SCOTTISH HIGHLANDS. 
163 
the zone under consideration, when clearly exposed, is everywhere charac- 
terized by large and peculiar Crustacea, no one of which has ever been 
found in the Lower Silurian rocks. 
In North America it has been long known, from the writings of Dekay, 
Harlan, and Hall, that both Eurypteri and Pterygoti occur in a black, so- 
called greywacke slate at Westmoreland in Oneida County, New York, which 
will probably be found to be on the parallel of the Upper Ludlow rock. And 
even in Canada, as we learn from the collections of Sir W. E. Logan, species of 
Eurypterus or Pterygotus occur. The discovery of the large Eurypterida 
in the same geological zone in other distant regions is therefore peculiarly 
satisfactory. 
Wherever these large Crustaceans are found, and with them small Lin- 
gulae and Spiral Shells, we may be sure that we are at or near the very 
summit of all rocks to which the term Silurian can be applied, and that 
the next overlying stratum belongs to the first great era of Pishes, the 
Devonian or Old Red Sandstone. 
Altered (Metamorphosed) Lower Silurian Rocks, and their Superposition 
to Cambrian and Laurentian Rocks, in the North-western Highlands. — In 
the preceding brief sketch of the older strata of the south of Scotland, I 
have made few allusions to the igneous rocks which have been intruded 
among the Silurian deposits. In numberless cases, however, whether around 
the granite of Criffel and Cairnsmuir, or the porphyry of Tongueland im- 
mediately to the north of Kirkcudbright, the schists which are in contact 
with such eruptive masses are so highly metamorphosed, that no one who 
looks at the effects can doubt as to their cause. 
The view of much more extended metamorphism in the North of Scot- 
land (due to a much grander cause) was indeed suggested by me in 1851 *. 
I then endeavoured to show that certain bands of clay-slate, and of chlo- 
ritic and micaceous schist, forming the southern zone of the Highlands, 
with their interstratified limestones, were probably nothing more than 
metamorphosed Lower Silurian rocks, similar in age to the unaltered strata 
of the South of Scotland, and which, ranging parallel to them, come out to 
the surface again in broad undulations, associated with many granitic and 
other eruptive rocks, to the north of the great central Caledonian trough of 
Old Red Sandstone and overlying coal-fields f . 
One of these great undulations, which occur in the rugged mountains of 
the west, called Argyll's Bowling- Green, exhibits an axis of mica- schist 
throwing off to the north and south great thicknesses of chloritic schist and 
altered sandstone, with included and regularly stratified limestones, all 
trending from W.S.W. to E.N.E., parallel to the Silurian rocks of the 
South of Scotland. 
In a subsequent memoir, Professor Nicol supported this idea, which he 
had long entertained, by striking facts and good reasoning. Describing 
* See Quart. Journ. Geol. Soc. vol. vii. p. 168. t Ibid. p. 160. 
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