LITIIOLOGY. 213 



ular and even planes, but the individual laminx are much bent and 

 twisted. A specimen from Rye contains numerous microscopic apatite 

 crystals, which are visible in thin sections. 



Biotite Gneiss. This gneiss is much more common. Like all our 

 gneisses, it varies in structure from the very schistose kinds to those 

 in which the lamination is invisible in small specimens, and only faintly 

 visible in large masses, and which are called gneiss granites. I will men- 

 tion only a few prominent peculiarities of individual specimens, none of 

 which are uncommon. A gneiss from Swanzey is very uniform in its 

 texture, and a thin section shows that its feldspar is largely oligoclase. 

 It would be called oligoclase gneiss. It contains garnet and epidote, and 

 also a few crystals of apatite. Some grains of oligoclase are twinned in 

 two directions, and the strise are very fine, making most beautiful objects 

 of the crystals. The biotite is very deeply colored, and strongly dichroic. 

 A gneiss that underlies the schists on the Saco river is very feldspathic, 

 but the feldspar is mostly orthoclase. It contains magnetite, and also 

 microscopic cubes of pyrites. A variety from Eagle cliff is pseudo-por- 

 phyritic in structure, some of the crystals of orthoclase being very large. 

 A specimen from Tripyramid is remarkable for the large number and 

 large size of the fluidal cavities in its quartz. It also contains large 

 grains of titanite. Another specimen from the same locality contains 

 numerous microscopic scales of hematite, which in the thin sections ap- 

 pear of a deep red color. A specimen from Holderness is very black on 

 account of the preponderance of biotite. A kind quarried at Enfield is 

 very light, because the amount of mica is small. A variety very coarse 

 in texture comes from Wolfeborough. A variety with a very dark green 

 shade comes from Whitefield, This shade appears to result from the 

 original color of the mica, and not, as it often does, from decomposition. 

 All these peculiarities are combined in various ways in other specimens. 



Biotite Muscovite Gneiss. This is the most common variety of gneiss, 

 and the larger number of our specimens are classed here. Prominent 

 in this group are the old pseudo-porphyritic gneisses that character- 

 ize the formations that are referred by Prof. Hitchcock to the Lauren- 

 tian. These gneisses are mostly dark in their color, since the biotite 

 preponderates over the muscovite. In them large orthoclase crystals, 

 which are often two inches long, are developed. These crystals are com- 



