66 HISTORICAL PALAEONTOLOGY. 



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). 



The Lozver 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 



Fig. 20 Diagrammatic section of the Laurentian Rocks In LowerCanada. a Lower 

 Laurentian; 6 Upper Laurentian, resting unconformably upon the lower series; cCam- 

 brian strata (Potsdam Sandstone), resting unconformably on the Upper Laurentian. 



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 is 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 localities, 

 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 Syke 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 

 ancient metamorphic rocks of Bohemia and Bavaria may be 

 regarded as being approximately of the same age. 



By some geological writers the ancient and highly meta- 



