XIII.] GEOGNOSY OF THE APPALACHIANS. 243 



be of organic origin, without, however, retracting their opinion 

 as to the palaeozoic age of the strata. Eeserving to another 

 place in my address the discussion of the geological age of the 

 White Mountain rocks, I proceed to notice briefly the distinc- 

 tive characters of the three groups of crystalline strata just 

 mentioned, which will be shown in the sequel to have an im- 

 portance in geology beyond the limits of the Appalachians. 



I. The Adirondack or Laurentide Series. The rocks of this 

 series, to which the name of the Laurentian system has been 

 given, may be described as chiefly firm granitic gneisses, often 

 very coarse grained, and generally reddish or grayish in color. 

 They are frequently hornblendic, but seldom or never con- 

 tain much mica, and the mica-schists (often accompanied with 

 staurolite, garnet, andalusite, and cyanite), so characteristic of 

 the White Mountain series, are wanting among the Laurentian 

 rocks. They are also destitute of argillites, which are found in 

 the other two series. The quartzites, and the pyroxenic and 

 hornblendic rocks, associated with great formations of crystal- 

 line limestone, with graphite, and immense beds of magnetic 

 iron-ore, give a peculiar character to portions of the Laurentian 

 system. 



II. The Green Mountain Series. The quartzo-feldspathic 

 rocks of this series are to a considerable extent represented by 

 a fine-grained petrosilex or eurite, though they often assume 

 the form of a true gneiss, which is ordinarily more micaceous 

 than the typical Laurentian gneiss. The coarse-grained, por- 

 phyritic, reddish varieties common to the latter are wanting in 

 the Green Mountains, where the gneiss is generally of pale 

 greenish and grayish hues. [The quartziferous porphyries, 

 which have been noticed ante, page 187, are supposed, in the 

 present state of our knowledge, to belong to this series.] Mas- 

 sive stratified diorites, and epidotic and chloritic rocks, often 

 more or less schistose, with steatites, dark-colored serpentines, 

 and ferriferous dolomites and magnesites also characterize this 

 gneissic series, and are intimately associated with beds of iron- 

 ore, generally a slaty hematite, but occasionally magnetite. 

 Chrome, titanium, nickel, copper, antimony, and gold are fre- 



