142 



AZOIC AGE. 



stone is represented in connection with granite and other rocks. In fig. 145, 

 I is limestone, without any appearance of stratification; and the containing 

 rock is granite. In fig. 146, a a are gneiss, b steatite, I unstratified limestone. 



Fig. 146. 



Although a and b are not evenly stratified, yet they are sufficiently so to show 

 that the limestone, while it has lost its division into layers in the crystallizing 

 process, is probably a conformable stratum. 



The order of stratification among the Azoic rocks is as various as 

 among the rocks of other ages. As sandstones, shales, argilla- 

 ceous sandstones, conglomerates, follow one another in any succes- 

 sion, so granite or gneiss may lie between layers of slate or schist, 

 and quartz rock may have any place in the series. It is common, 

 however, to find the different hornblendic rocks associated toge- 

 ther ; and both these and the chloritic often abound in the iron- 

 regions, since hornblende and chlorite are ferriferous minerals. 

 Again, chloritic schists are apt to accompany serpentine ; since 

 chlorite is a hydrous magnesian species. 



Again, as we recede from a granite region we sometimes find the 

 rocks less and less perfectly crystalline, passing from the granite 

 to the gneissoid, from these to the schistose, and last to those least 

 crystalline and containing water, as talcose and chloritic schist 

 and serpentine. But there are numberless exceptions to such an 

 order. 



The Azoic rocks of Canada are divided by Logan into the Laurentian, — 

 including the great part of the system, and embracing all the regions to which 

 we have above particularly alluded, — and the Huronian, comprising a narrow 

 band on the borders of Lake Superior and Lake Huron. The Huronian are 

 thus separated because they have been found to rest unconformably upon the 

 Laurentian. They consist of siliceous slates, schists, quartzite, conglomerates, 

 and limestones. The conglomerates contain pebbles and boulders (some a foot 

 in diameter), which are derived in part from gneiss or syenite of the subjacent 

 Azoic, and thus show their later origin. Others contain pebbles of jasper and 

 quartz very firmly cemented. The sandstones are described also as bearing 

 ripple-marks. These rocks occur on Lake Temiseaming and on the north shore 

 of Lake Huron. No similar rocks have been observed to the eastward of these 

 districts, over all Canada to Labrador; and if ever there, they have been re- 

 moved by denudation. The Huronian rocks are intersected by numerous dikes 

 of trap, and interstratified and overlaid by the same rock. The total thickness 

 of the formation, according to Logan, is over twelve thousand feet. The iron- 



