2/8 



STRATIGRAPHICAL GEOLOGY, 



y cj 



Lisbon, to Smith's brook in Lyman, giving, also, the thicknesses as meas- 

 ured on this hne. 



At the south-east end of the section there are the ordinary gneisses of 

 the Lake division, dipping about N. 30° W., and holding a band of azoic 

 K CO- ^ limestone, perhaps one hundred feet thick, inclined 

 50°, which has been extensively quarried by Mr. Orrin 

 Bronson. On the hillside toward the pond are fria- 

 ble gneisses, often very micaceous and carrying crys- 

 tals of staurolite, dipping 30° N. 70° W. This is bor- 

 dered by a band of hornblende schist dipping in the 

 same direction : 40° is the average for its whole width. 

 ^ The hornblende is an associate of the gneiss forma- 

 H tion rather than of that which follows. The estimated 

 .g thickness of this gneissic group, between the lime- 

 ^-. stone and Mink pond, is 2500 feet. This assemblage 

 ^" of gneisses and associated rocks west of the limestone 

 ^ is slightly suggestive of the Montalban member of the 

 I Atlantic system. On the north side of Mink pond is 

 I a gray, friable mica schist, holding in profusion the 

 ^ reddish staurolites and garnets, the locality being one 

 M well known to mineralogists. The average dip being 

 '> 56°, the thickness must be 3300 feet. This band is 



1 6 followed by the same garnetiferous slates which occur 



2 I upon the south branch of the Ammonoosuc. Stauro- 

 ■■ .5 

 s''^^ lite is less abundant in this than in the previous band, 



" " and it is almost wanting in the western portion. With 



J s '^ ^ 



;3 I an average dip of 58°, this slate must be over 3000 



"^ '^ feet in thickness. This all belongs to the Coos group. 



e ^ The Swift Water series follows. For one hundred 



.X I and forty rods there are quartzitcs and sandstones, 



■f, I with an average dip of 50°, which gives 1769 feet in 



t' J thickness. More particularly, an excavation for gold 



^- r. at the eastern border shows slaty layers, quite sili- 



li ccous, with sandstones considerably vitrified. Next is 



1 1 a sandstone, with whitish cement. Then there are in 



= i order actinolite schist, hornblende schist, while mica 



