HIGHLAND METAMORPHIC ROCKS. 
601 
series lies uncoiiformably upon the Lower, and differs from it 
chiefly in having as y^t yielded no fossils. It consists of 
gneiss with Labrador-feldspar and feldstones, in all 10,000 feet 
thick, and both its composition and structure lead us to sup¬ 
pose that, like the Lower Laurentian, it was originally of 
sedimentary origin and owes its crystalline condition to met- 
amorphic action. The remote date of the period when some 
of these old Laurentian strata of Canada were converted 
into gneiss may be inferred from the fact that pebbles of that 
rock are found in the overlying Huronian formation, which is 
probably of Cambrian age (p. 490). 
The oldest stratified rock of Scotland is the hornblendic 
gneiss of Lewis, in the Hebrides, and that of the north-west 
coast of Ross-shire, represented at the base of the section 
given at Fig. 82, p. 112. It is the same as that intersected 
by numerous granite veins which forms the cliffs of Cape 
Wrath, in Sutherlandshire (see Fig. 613, p. 560), and is con¬ 
jectured to be of Laurentian age. Above it, as shown in 
the section (Fig. 82, p. 112), lie unconformable beds of a red¬ 
dish or purple sandstone and conglomerate, nearly horizon¬ 
tal, and between 3000 and 4000 feet thick. In these ancient 
grits no fossils have been found, but they are supposed to be 
of Cambrian date, for Sir R. Murchison found Lower Silurian 
strata resting unconformably upon them. These strata con¬ 
sist of quartzite with annelid burrows already alluded to (p. 
112), and limestone in which Mr. Charles Peach was the first 
to find, in 1854, three or four species of Orthoceras^ also the 
genera Cyrtoceras and Lituites^ two species of Murchisonia^ 
a Pleurotomaria^ 2 i species of Macliirea^ of Euomphalus^ 
and an Orthis. Several of the species are believed by Mr. 
Salter to be identical with Lower Silurian fossils of Canada 
and the United States. 
The discovery of the true age of these fossiliferous rocks 
was one of the most important steps made of late years in 
the progress of British Geology, for it led to the unexpected 
conclusion that all the Scotch crystalline strata to the east¬ 
ward, once called primitive, which overlie the limestone and 
quartzite in question, are referable to some part of the Silu¬ 
rian series. 
These Scotch metamorphic strata are of gneiss, mica-schist, 
and clay-slate of vast thickness, and having a strike from 
north-east to south-west almost at right angles to that of 
the older Laurentian gneiss before mentioned. The newer 
crystalline series, comprising the crystalline rocks of Aber¬ 
deenshire, Perthshire, and Forfarshire, were inferred by Sir 
R. Murchison to be altered Silurian strata; and his opinion 
26 
