498 Prof. T. Sterry Sunt — Elements of Primary Geology. 



For this reason the often used word Archaean, from its very vagueness 

 and indefiniteness, is perhaps, in the present state of our knowledge, 

 the most convenient term to designate the more or less completely- 

 crystalline or colloidal rocks which preceded the essentially detrital 

 rocks of Secondary time, and by their decay and disintegration 

 furnished the chief part of the material of these. 



12, As a result of all these considerations, the author has been led 

 to attempt a subdivision of the Archagan indigenous rocks into 

 several chronological groups, which he has often described elsewhere, 

 and which, in his Mineral Physiology and Physiography, are concisely 

 treated in a chapter entitled A History of Pre-Cambrian Eocks. It 

 will therefore be sufficient for the present purpose to recall briefly 

 the great subdivisions there advocated. 



I. Under the name of Laurentian are comprised the gneissic rocks 

 of the Laurentides, the Adirondacks, the Highlands of the Hudson, 

 and much of the Eocky Mountains in North America. These rocks 

 are divided, so far as known, into a lower granitoid series, often but 

 obscurely stratified, and without included limestones or quartzites 

 (the Ottawa gneiss) and an apparently unconformable series of 

 gneisses, much resembling the last, but with interbedded quartzites, 

 limestones and iron-oxyds (the Grenville series). The name of 

 Laurentian was originally (in 1854) made to include both of these, 

 without regard to the subdivisions ; which were, however, pointed 

 out in 1847, and which we may provisionally designate as Lower 

 and Upper Laurentian. The latter term, afterwards applied by 

 Logan to a succeeding series, otherwise called Labradorian, having 

 been superseded by Norian, the name of Middle Laurentian, which 

 some have erroneously applied to the Grenville series, or upper 

 division of the Laurentian, becomes meaningless.^ 



II. The Norian series, in which stratiform crystalline rocks, 

 granitoid in structure, nearly or quite free from quartz, and com- 

 posed essentially of basic feldspars, of which labradorite is the type, 

 constituting the rocks known as norite, is our next great division. 

 It, however, includes also occasionally quartzose and gneissic 

 rocks, together with crystalline limestones, and iron-oxyds, generally 

 titaniferous. From the resemblance of these gneisses to those of 

 the Laurentian, and from the few contacts observed, this Norian 

 division is assigned the second place in the succession. The 

 characteristic rocks of this series are often called gabbro, but are 

 widely distinct from the euphotide of the Huronian or Pietre-verdi 

 series, to which this name was first applied. 



III. At the base of the Huronian appears in many regions a great 

 series essentially composed of petrosilex, often becoming a quartz- 

 iferous porphyry, and occasionally interstratified with an indigenous 



1 Inasmuch as these distinctions were first devised and announced hy the Canadian 

 Geological Survey, it may here be said that its earliest printed reports on the ancient 

 rocks of the country (those for 1845 and 1846, published in 1847) were prepared from 

 the notes and collections of Logan and Murray, hy the present writer after his 

 connection with that Geological Survey in February, 1847, and that all subsequent 

 matters relating to lithology in its reports up to 1872, were either prepared by him 

 or under his direction. 



