The Eozoic Rocks of North America. 83 



composed in large part of plagioclase feldspars, chiefly labradorite. 

 These three divisions of the Eozoic system were thus confounded 

 under the common name of Laurentian until, in 1862, the last was 

 separated under the provisional name of Upper Laurentian, the 

 two other divisions united being called Lower Laurentian. The 

 synonym of Labradorian was subsequently, for a time, employed 

 by Logan to designate the upper division, until 1870, when the 

 present writer proposed for it the name of Norian, retaining that 

 of Laurentian for the two lower divisions. It will probably be 

 found desirable to separate the typical Laurentian or Grenville 

 series, as studied and mapped by Logan, Hunt and Dawson, from 

 the less-known fundamental or Ottawa gneiss, and to make of this 

 latter a distinct gronp. The name of Middle Laurentian, some" 

 times given to the typical Laurentian, loses its significance with the 

 disappearance of that of Upper Laurentian, now replaced by Norian- 



The Norian series is made up in great part of granitoid or gneis- 

 soid rocks, composed essentially of plagioclase feldspars, without 

 quartz but with a little pyroxene or hypersthene, often with titanic 

 iron ore, and apparently identical with the norites of Norway. 

 With these rocks are, however, found alternations of gneiss, of 

 quartzite and of crystalline limestone, scarcely different from those 

 of the Laurentian. We therein find also a granitoid rock made up 

 of pink orthoclase and quartz, with bluish labradorite. This Norian 

 series is found in many places covering considerable areas, and 

 apparently resting in discordant stratifications upon the typical 

 Laurentian. Its thickness has been estimated at over 10,000 feet. 



There is found in certain localities a series of stratified rocks, 

 composed essentially of petrosilex or halleflinta, often passing into 

 a quartziferous porphyry. There are found with it strata of 

 vitreous quartzite, and thin layers of soft micaceous schists, besides 

 great beds of hematite, and more rarely layers of crystalline lime- 

 stone. This group, which has a thickness of many thousand feet 

 was at first included by the writer in the lower part of the succeed- 

 ing Huronian series, which, however, apparently overlies it uncon- 

 formably. Its relations with the preceding groups have not been 

 observed, but, as it appears to be identical, both in position and 

 in character, with what, in 1878, was first called Arvonian in 

 Wales, we designate it by that name. 



Next in order comes the group to which the writer, in 1855, 



