As regards its mineral characters, the Laurentian series is composed
throughout of metamorphic and highly crystalline rocks, which
are in a high degree crumpled, folded, and faulted. By the late
Sir William Logan the entire series was divided into two great
groups, the _Lower Laurentian_ and the _Upper Laurentian_, of
which the latter rests unconformably upon the truncated edges
of the former, and is in turn unconformably overlaid by strata
of Huronian and Cambrian age (fig. 20).
[Illustration: 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.
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