tration: Fig. 20.--Diagrammatic section of the Laurentian
Rocks in Lower Canada. a Lower Laurentian; b Upper Laurentian,
resting unconformably upon the lower series; c Cambrian strata
(Potsdam Sandstone), resting unconformably on the Upper Laurentian.]

The _Lower Laurentian_ series attains the enormous thickness of
over 20,000 feet, and is composed mainly of great beds of gneiss,
altered sandstones (quartzites), mica-schist, hornblende-schist,
magnetic iron-ore, and haematite, together with masses of limestone.
The limestones are especially interesting, and have an extraordinary
development--three principal beds being known, of which one is
not less than 1500 feet thick; the collective thickness of the
whole being about 3500 feet.

The _Upper Laurentian_ series, as before said, reposes unconformably
upon the Lower Laurentian, and attains a thickness of at least
10,000 feet. Like the preceding, it is wholly metamorphic, and
is composed partly of masses of gneiss and quartzite; but it
is especially distinguished by the possession of great beds of
felspathic rock, consisting principally of "Labrador felspar."

Though typically developed in the great Canadian area already
spoken of, the Laurentian Rocks occur in other localities, both
in America and in the Old World. In Britain, the so-called
"fundamental gneiss" of the Hebrides and of Sutherlandshire is
probably of Lower Laurentian age, and the "hypersthene rocks"
of the Isle of Skye may, with great probability, be regarded
as referable to the Upper Laurentian. In other localities in
Great Britain (as in St David's, South Wales; the Malvern Hills;
and the North of Ireland) occur ancient metamorphic deposits
which also are probably referable to the Laurentian series. The
so-called "primitive gneiss" of Norway appears to belong to the
Laurentian, and the ancient metamorphic rocks of Bohemia and
Bavaria may be regarded as being approximately of the same age.

[Illustration: Fig. 21.--Section of Lower Laurentian Limestone
from Hull, Ottawa; enlarged five diameters. The rock is very
highly crystalline, and contains mica and other minerals. The
irregular black masses in it are graphite. (Original.)]

By some geological writers the ancient and highly metamorphosed
sediments of the Laurentian and the succeeding Huronian series
have been spoken of as the "Azoic rocks" (Gr. _a_, without; _zoe_,
life); but even if we were wholly destitute of any evidence of
life during these