66 HISTORICAL PALEONTOLOGY. 



timbered, but rarely well fitted for agriculture, and chiefly 

 attractive to the hunter and the miner. 



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 uncon- 

 formably overlaid by strata of Huronian and Cambrian age 

 (fig. 20). 



'^\\& Lower Laurentian series attains the enormous thickness of 



Fig. 20. Diagrammatic section of the Laurentian Rocks in Lower Canada, a Lower 

 Laurentian ; b 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 haematite, 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 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 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 



