ON THE CHEMISTRY OP METAMORPHIC ROCKS. 205 



IV. In the fourth place are to be noticed the metamorphosed 

 strata of Upper Silurian and Devonian age, with which may also be 

 included those of the Carboniferous system in eastern New England. 

 This group has as yet been imperfectly studied, but presents inter- 

 esting peculiarities. 



In the oldest of these, the Laurentian system, the first class of 

 aluminous rocks takes the form of granitoid gneiss, which is often 

 coarse grained and porphyritic. Its feldspar is frequently a nearly 

 pure potash orthoclase, but sometimes contains a considerable pro- 

 portion of soda. Mica is often almost entirely wanting, and is 

 never abundant in any large mass of this gneiss, although small 

 bands of mica-schist are occasionally met with. Argillites, which 

 from their general predominance of potash and of silica, are related 

 to the first class of sediments, are, so far as known, wanting through- 

 out the Laurentian series ; nor is any rock here met with, which 

 can be regarded as derived from the metamorphism of sediments 

 like the argillites of more modern series. Chloritic and chiastolite 

 schists, and kyanite are, if not altogether wanting, extremely rare 

 in the Laurentian system. The aluminous sediments of the second 

 class are however represented in this system by a diabase made 

 up of dark green pyroxene and bluish labradorite, often associated 

 with a red aluraino-ferrous garnet. This latter mineral also some- 

 times constitutes small beds, often with quartz, and occasionally 

 with a little pyroxene. These basic aluminous minerals form how- 

 ever but an insignificant part of the mass of strata. This system 

 is farther remarkable by the small amount of ferruginous matter 

 diffused through the strata; from which the greater part of the 

 iron seems to have been removed, and accumulated in the form of 

 immense beds of hematite and magnetic iron. Beds of pure 

 crystalline plumbago also characterize this series, and are generally 

 found with the limestones. These are here developed to an extent 

 unknown in more recent formations; and are associated with beds 

 of crystalline apatite, which sometimes attain a thickness of several 

 feet. The serpentines of this series, so far as yet studied in Canada, 

 are generally pale colored, and contain an unusual amount of 

 water, a small proportion of oxide of iron, and neither chrome nor 

 nickel ; both of which are almost always present in the serpentines 

 of the third series. 



The second or Labrador series is characterized, as already re- 

 marked, by the predominance of great beds of anorthosite, com- 

 posed chiefly of triclinic feldspars, which vary in composition from 

 auorthite to andesine. These feldspars sometimes form mountain 



