124 



These latter are ordinarily light-colored from the presence of 

 white feldspar, and, though generally fine in texture, are 

 sometimes coarse-grained and porphyritic. They are less 

 strong and coherent than the gneisses of the Laurentian, and 

 pass, through the predominance of mica, into mica-schists, 

 which are themselves more or less tender and friable, and 

 present every variety, from a coarse, gneiss-like aggregate down 

 to a fine-grained schist, which passes into argillite. . . . The 

 White Mountain rocks also include beds of micaceous quartz- 

 ite. The basic silicates in this series are represented chiefly 

 by dark-colored gneisses and schists, in which hornblende takes 

 the place of mica. These pass occasionally into beds of dark 

 hornblende rock, sometimes holding garnets. Beds of crystal- 

 line limestone occasionally occur in the schists of the White 

 Mountain series, and are sometimes accompanied by pyroxene, 

 garnet, idocrase, ephene, and graphite. . . . The limestones 

 are intimately associated with the highly micaceous schists con- 

 taining staurolite, andalusite, cyanite, and garnet. These 

 schists are sometimes highly plumbaginous. . . . To this third, 

 or White Mountain, series of crystalline schists belong the con- 

 cretionary granitic veins abounding in beryl, tourmaline, and 

 lepidolite, and occasionally containing tinstone and columbite." 

 Although designed merely as a general statement of the char- 

 acteristics of the Montalban formation, applicable to the series 

 as a whole in its widest distribution, and without special refer- 

 ence to any particular region, no student of experience in the 

 geology of Massachusetts will fail to recognize the foregoing 

 brief synopsis as a singularly comprehensive and accurate enu- 

 meration of the rocks composing the areas designated above as 

 Montalban. But if we would include in the Montalban forma- 

 tion all the rocks properly belonging there, according to the 

 above definition of the system, it will be necessary, I think, to 

 extend the limits which Dr. Hunt has assigned this series in 

 Massachusetts. In 1870,^ and again in the following year,^ 

 this authority referred the limestone in Chelmsford containing 



1 Amer. Jour. Sci., ii., xlix., 75. 



' Chemical and Geological Essays, p. 249. 



