152 
SILUKIA. 
[Chap. VIII. 
legs successively produced the imprints (not four or five or more, as in the 
Canadian tracks) ; and they are most likely to have been made by an animal 
swimming with difficulty in very shallow water, such as Prof. Harkness 
shows from physical evidence to have been the condition of the locality 
(Quart. Geol. Journ. vol. xii. p. 243). The reader will find in the fifteenth 
Chapter, which treats of the Continental rocks of Silurian age, that the 
lowest fossiliferous rocks of the Thiiringerwald contain Protovirgularia 
associated with Nereites. 
Although the axial dome, a, of purple rocks, for the most part arena- 
ceous, throws off schists to the south as well as to the north, and though on 
both sides anthracite and traces of Graptolites have been found in the beds 
marked h 1 , the Silurian rocks are soon lost in the former direction under 
the red sandstone (Permian) north of the Solway Prith, or the Old Red 
and Carboniferous formations or porphyries of the Cheviot Hills. To the 
north of the axis of the Teviot, a long ascending section is presented to the 
observer, though it must be presumed that there are many flexures which 
are not marked in the diagram. In this direction the schists and grey- 
wacke, b 1 , which are in parts alum-slates (Etterick), may represent in 
time, as suggested by Professor Nicol, the schists of North Wales with 
Lingulse, though as yet the Scotch strata have afforded no such fossils. 
These are surmounted by other schists, here and there containing thin la- 
minae of anthracite, 6 2 , which formerly gave rise to the notion of coal being 
subordinate to these rocks. It has been surmised that this anthracite re- 
sulted from the carbonization of Sea-weeds, or possibly of Graptolites and 
Annelides ; but the discovery by Nicol of some imperfect reed-like Plants 
in the rock, with a minute vascular or tubular structure in the burnt 
residue of the anthracite, has led to the suggestion that some sort of grassy 
vegetation existed on the adjacent lands, or some Zostera-like plants in the 
sea, during this ancient period. 
The anthracitic schists, b 2 , contain a few Phyllopod Crustacea f and many 
so-called Annelide-markings ; they form the base of a vast thickness of 
graptolite-schists, which, after a synclinal flexure in the Yarrow Hills, 
show another anticlinal in the valley of the Tweed, 6*. Whilst the Thor- 
nielee slates, with their Graptolites and Annelide-marks (Nereites, Crosso- 
podia) &c, are inclined southward, the Grieston slates, on the north bank 
of the anticlinal, plunge to the north, and then for the first time in ascend- 
ing order we find Trilobites in addition to the Graptolites and Annelides. 
Associated with felspar-porphyries (the whole representing the trappean 
and graptolitic group of North and South Wales), these rocks rise into 
mountains 2200 feet above the sea; and from them the sources of the 
Clyde, as well as of the Tweed, take their rise. Thence, the northern dip 
being continued, the whole of the preceding masses are seen to be overlain 
t See Quart. Journ. Geol. Soc. vol. viii. p. 391. Woodward has recently described another genus 
A larger species than those referred to has since of these Crustacea from the same shales (Quart, 
been found by Professor Harkness ; and Mr. H. Journ. Geol. Soc. vol. xxii. p. 503). 
