32 THE CHEMISTRY OF METAMORPHIC ROCKS. [III. 



Adirondack Mountains ; of which, according to Enunons, they 

 form the highest summits.* 



In the third (or Green Mountain) series, which we have 

 referred to the Lower Silurian age, the gneiss is sometimes 

 granitoid, but less markedly so than in the first; and it is 

 much more frequently micaceous, often passing into micaceous 

 schist, a common variety of which contains disseminated a 

 large quantity of chloritoid. Argillites abound, and under 

 the influence of metamorphism sometimes develop crystalline 

 orthoclase. At other times they are converted into a soft 

 micaceous mineral, and form a kind of mica-schist. Chias- 

 tolite and staurolite are never met with in the schists of this 

 series, at least in its northern portions, throughout Canada and 

 New England. The anortholites of the Labrador series are 

 here represented by fine-grained diorites, in which the feldspar 

 varies from albite to very basic varieties, which are sometimes 

 associated with an aluminous mineral allied to chlorite in com- 

 position. Chloritic schists, frequently accompanied by epidote, 

 abound in this series. The great predominance of magnesia 

 in the forms of dolomite, magnesite, steatite, and serpentine, 

 is also characteristic of portions of this series. The latter, 

 which forms great beds (ophiolites), is marked by the almost 

 constant presence of small portions of the oxides 6f chrome 

 and nickel. These metals are also common in the other rnag- 

 nesian rocks of the series ; green chrome-garnets, and chrome- 

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

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

 in eastern North America, and contains large quantities of 



copper ores in interstratified beds The only graphite. 



which has been found in the third series is in the form of 

 impure plumbaginous shales. 



The metamorphic rocks of the fourth (or White Mountain) 

 series, as seen in southeastern Canada, are for the greater part 

 quartzose and micaceous schists, more or less feldspathic ; 

 which in certain portions become remarkable for a great 



* A further description of this Labrador or Norian series is given in Essay 

 XIII. 



