30 THE CHEMISTRY OF METAMORPHIC ROCKS. [III. 



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 stud- 

 ied, but presents interesting 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 fre- 

 quently a nearly pure potash-orthoclase, but sometimes con- 

 tains a considerable proportion 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 silica are related to the first class of sediments, 

 are, so far as known, wanting throughout the Laurentian 

 series ; nor is any rock here met with, which can be regarded 

 as derived from the metamorphism of sediments like the argil- 

 lites 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 alumino-ferrous garnet. This latter 

 mineral also sometimes constitutes small beds, often with 

 quartz, and occasionally with a little pyroxene. These basic 

 aluminous minerals form, however, but an insignificant part 

 of the mass of strata. This system is further 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 and veins of crystalline 

 plumbago also characterize this series, and are generally found 

 with the limestones, which are here developed to an extent 



while attempting to describe but four ; the fifth being the Huronian series, 

 which from its close resemblance to the third series (from which it was by 

 Logan regarded as geologically distinct), was to me a source of great per- 

 plexity. For further considerations touching this question, see the remarks 

 on page 18.] 



