Dr. T. Sterry mint—The Eozoic Rocks of N. America. 507 



intercalations of liornblendic gneiss, of quartzite, of pyroxenite, of 

 serpentine, of magnetite, and especially of crystalline limestones, 

 the latter often magnesian, occasionally graphitic, and sometimes 

 attaining thicknesses of a thousand feet or more. This Grenville 

 series, the strata of which are generally highly inclined, has an 

 aggregate volume of not less than 15,000 or 20,000 feet, and appears 

 to rest unconformably upon the fundamental or Ottawa gniess. This 

 gneissic series, with its intercalated limestones, some of which contain 

 Eozooii Canadense, was the typical Laurentian of Logan and Hunt, 

 named by them in 1854, with which they included, at that time, 

 however, not only the underlying fundamental gneiss, but an upper 

 granitoid and gneissoid series composed in large part of plagioclase 

 felspars, 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 group. The name of Middle Laurentian sometimes given 

 to the typical Laurentian loses its significance with the disappear- 

 ance of that of Upper Laurentian, now replaced by Norian. 



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

 rocks composed essentially of plagioclase felspars, 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 stratification 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 essentiallj'^ 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 limestone. 

 This group, which has a thickness of many thousand feet, was at 

 first included by the writer in the lower part of the succeeding 

 Huronian series, which, hovvever, apparently overlies it unconform- 

 ably. 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 called Arvonian in Wales, we 

 designate it by that name. 



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

 the name of Huronian. It differs from those preceding by the 



