656 STRATIGRAPHICAL GEOLOGY. [Boox VL 
of which 8 genera and 9 species pass down into the Lingula flags and 9 
genera and 13 species ascend into the Upper Tremadoc zone. The most 
characteristic forms are Niobe Homfrayi, N. menapiensis, Psilocephalus 
imnotatus, Angelina Sedqwickii, Asaphus affinis. 'The Upper Tremadoc 
beds contain, as at present ascertained, 20 genera and 33 species, of 
which only 16 species pass up into the Arenig group. It is at the top of 
the Upper Tremadoc strata that the line between the Cambrian and 
Silurian systems is here drawn. According to Sir A. C. Ramsay, there 
is evidence-of a physical break at the top of the Tremadoc beds of Wales, 
so that on a large scale the next succeeding or Arenig strata repose un- 
conformably upon everything older than themselves; while Mr. Etheridge 
remarks that no greater break in paleontological succession occurs in 
the whole series of Paleeozoic rocks than at this point, for besides the 
small percentage of fossils of the one series that passes over into the other 
(16 species in all) the character of the Arenig fauna strongly distin- 
guishes it from that of the formations below, and further supports the 
line of division here adopted between the Cambrian and Silurian rocks. 
But, as already remarked, the demarcation does not interfere with the 
broad general resemblance in the paleontological facies of the two 
systems. Unfortunately in England, where the question has been 
principally discussed, personal considerations have been allowed to in- 
fluence the judgment, the partizans of Sedgwick on the one hand and 
of Murchison on the other contending for the claims of the rival geo- 
logical chiefs. When the personal element can be entirely eliminated, 
and the question is discussed on its own merits, the line of demarcation 
between Cambrian and Silurian, as above suggested, will not improbably 
be effaced, and the whole will be regarded as one great paleontological 
system. 
A In the north-west of Scotland a mass of reddish-brown and chocolate- 
coloured sandstone and conglomerate (at least 8000 feet thick in the 
Loch Torridon district) lies unconformably upon the Archean gneiss 
in nearly horizontal or gently inclined beds. It rises into picturesque 
groups of mountains, which stand out as striking monuments of denuda- 
tion, seeing that the truncated ends of their component flat strata can be 
traced even from a distance forming parallel bars along the slopes and 
precipices. ‘The denudation must have been considerable even in early 
Silurian times, for the sandstones are unconformably overlaid by quartz- 
rocks and limestones containing Lower Silurian fossils, and these 
younger strata even in the same district rest directly on the Archean 
eneiss. Here and there at the base of the red sandstone lies a 
remarkably coarse breccia containing huge angular blocks of gneiss. At 
these localities rounded dome-like bosses of gneiss pass under the 
breccia and forcibly recall the roches moutonnées of more recent times} 
No trace of organic remains of any kind has been found in the red sand- 
stones themselves, unless certain track-like impressions, observed on the 
west side of Loch Maree, can be regarded as having been imprinted by 
crustacea or other organisms.” hese sandstones were at one time 
regarded as Old Red Sandstone, though Macculloch, and afterwards Hay 
Cunningham, pointed out that they underlie parts of the schistuse rocks 
of the northern Highlands. ‘The discovery by Mr. C. W. Peach of Lower 
Silurian shells in the overlying limestones showed that the massive red 
1 Nature, August (1880) xxii. p. 400, 2 Nature, xxiii, p. 98. 
