66 HISTORICAL PAL/ZEONTOLOGY. 
timbered, but rarely well fitted for agriculture, and chiefly 
attractive to the hunter and the miner. 
As regards its mineral characters, the Laurentian series 1s 
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 uncon- 
formably overlaid by strata of Huronian and Cambrian age 
(fig. 20). 
The Lower Laurentian series attains the enormous thickness of 
Fig. 20.—Diagrammatic section of thé Laurentian Rocks in Lower Canada. a@ Lower 
Laurentian; 4 Upper Laurentian, resting unconformably upon the lower series; c Cam- 
brian strata (Potsdam Sandstone), resting unconformably on the Upper Laurrentian. 
over 20,000 feet, and is composed mainly of great beds of gneiss, 
altered sandstones (quartzites), mica-schist, hornblende-schist, 
magnetic iron-ore, and hzmatite, together with masses of lime- 
stone. ‘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 uncon- 
formably upon the Lower Laurentian, and attains a thickness 
of at least 10,000 feet. Like the preceding, it is wholly meta- 
morphic, and 1s composed partly of masses of gneiss and quartz- 
ite ; 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 locali- 
ties, both in America and in the Old World. In Britain, the 
so-called ‘fundamental gneiss” of the Hebrides and of Suther- 
landshire is probably of Lower Laurentian age, and the “ hy- 
persthene rocks” of the Isle of Skye may, with great proba- 
bility, 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 refer- 
able to the Laurentian series. The so-called “ primitive gneiss” 
of Norway appears to belong to the Laurentian, and the 
