ON THE CHEMISTRY OF METAMORPHIC ROCKS. 207 



the other magnesian rocks of the series ; green chrome-garnets, and 

 chrome-mica occur; and beds of chrome iron ore are found in the 

 ophiolites of the series. It is also the gold-bearing formation of 

 eastern North America, and contains large quantities of copper 

 ores in interstratified beds resembling those of the Permian schists of 

 Mansfeld and Hesse. In some parts of this series pure limestones 

 occur, which contain various crystalline minerals common also to 

 the Lauren tian limestones, and to those of the fourth series. The 

 only graphite which has been found in the third series, is in the 

 form of impure plumbaginous shales. 



The metamorpiiic rocks of the fourth series, as seen in south- 

 eastern Canada, are for the greater part quartzose and mica- 

 ceous schists, more or less feldspathic; which in the neighboring 

 States become remarkable for a great development of crystals of 

 staurotide and of red garuet. A large amount of argillite occurs in 

 this series; and when altered, whether locally by the proximity of 

 intrusive rock, or by normal metamorphism, exhibits a micaceous 

 mineral, and crystals of andalusite ; so that it becomes known 

 as chiastolite slate in its southern extension. Granitoid gneiss is still 

 associated with these crystalline schists. Gold is not confined to 

 the third series, but is also met with in veins cutting the argilites 

 of Upper Silurian age. The crystalline limestones and ophiolites of 

 eastern Massachusetts, which are probably of this series, resemble 

 those of the Laurentian system ; and the coal beds in that region 

 are in some parts, changed into graphite. It is to be remarked 

 that the metamorphic strata of the third and fourth series are 

 contiguous throughout their extent, so far as examined, but are 

 everywhere separated from the Laurentian and Labrador series by 

 a zone of unaltered palaszoic rocks. 



Large masses of intrusive granite occur among the crystalline 

 strata of the fourth series, but are rare or unknown among the 

 older metamorphic rocks in Canada. The so-called granites of the 

 Laurentian and Lower Silurian appear to be in every case indigen- 

 ous rocks ; that is to say, strata altered in situ, and still retaining 

 evidences of stratification. The same thing is true with regard to 

 the ophiolites and the anorthosites of both series ; in all of which the 

 general absence of great masses of unstratified rock is especially 

 noticeable. No evidences of the hypothetical granitic substratum 

 are met with in the Laurentian system, although this is in one district 

 penetrated by great masses of syenite, orthophyre, and dolerite. 

 Granitic veins, with minerals containing the rarer elements, such as 



