204 ON THE CHEMISTRY OP METAMORPHIC ROCKS. 



In eastern North America, the crystalline strata, so far as yet 

 studied, maybe conveniently classed in five groups, corresponding 

 to as many different geological series, four of which will be con- 

 sidered in the present paper. 



I. The Laurentian system represents the oldest known rocks of 

 the globe, and is supposed to be the equivalent of the Primitive 

 Gneiss formation of Scandinavia, and that of the Western Islands 

 of Scotland, to which also the name of Laurentian is now applied. It 

 has been investigated in Canada along a continuous outcrop from 

 the coast of Labrador to Lake Superior, and also over a consider- 

 able area in northern New York. 



II. Associated with this system is a series of strata characterized 

 by a great development of anorthosites,of which the hypersthenite, or 

 opalescent feldspar-rock of Labrador, may be taken as a type. 

 These strata overlie the Laurentian gneiss, and are regarded as 

 constituting a second and more recent group of crystalline rocks, 

 to which the name of the Labrador series may be provisionally 

 given. From evidence recently obtained, Sir William Logan con- 

 ceives it probable that this series is unconformable with the older 

 Laurentian system, and is separated from it by a long interval 

 of time. 



III. In the third place is a great series of crystalline schists, 

 which are in Canada referred to the Quebec group, an inferior 

 part of the Lower Silurian system. They appear to correspond 

 both lithologically and stratigraphically with the Schistose group 

 of the Primitive Slate formation of Norway, as recognized by 

 Naumann and Keilhau, and to be there represented by the strata 

 in the vicinity of Drontheim, and those of the Dofrefeld. The 

 Huronian series of Canada in like manner appears to correspond 

 to the Quartzose group of the same Primitive Slate formation (5). 

 It consists of sandstones, imperfect varieties of gneiss, diorites, 

 silicious and feldspathic schists passing into argillites, with lime- 

 stones, and great beds of hematite. Though more recent than the 

 Laurentian and Labrador series, these strata are older than the 

 Quebec group ; yet from their position to the westward of the 

 greatest accumulation of sediments, they have been subjected to a 

 less complete metamorphism than the palaeozoic strata of the 

 East. The Huronian series is as yet but imperfectly studied, and 

 for the present will not be further considered. 



(5) See Macfarlane — Primitive Formations of Norway and. Canada 

 compared — Canadian Naturalist, vii., 113, 162. 



