LITHOLOGY. 195 
of laminz of white mica, in two determinate directions. The same lam- 
inze appear in the quartz, but there they lie scattered at hap-hazard 
through its mass. This granite contains a large proportion of quartz. 
The Marlborough granite is in all microscopic characteristics the same, 
but it is gneissoid in structure; and, indeed, it is said that all of these 
Concord granites show indications of a gneissoid structure in the 
quarries, although in specimens no structure whatever is to be seen. 
The Troy granite is like the Concord, but is lighter in color, and its thin 
sections are remarkable for the large proportion of microcline they con- 
tain. The structure of this microcline is very beautiful, and often its 
laminee are of extreme fineness. Fig. 6 on Pl. 10 represents a section of 
the Troy granite. A grain of microcline is in the middle. The quartz 
shows its characteristic enclosures, consisting of cavities, needles of 
rutile, and mica. The light muscovite is seen on the right, the dark bio- 
tite on the left, and the orthoclase above. The Roxbury granite is char- 
acterized like the Troy by much microcline. The multitude of minute 
though very perfect muscovite crystals that lie scattered through some 
of the grains of quartz is noticeable. Fig. 2 on Pl. 5 is drawn from a 
section of this granite (see p. 86). Other specimens representing the 
formation of so-called Concord granites offer but little in addition. A 
specimen from Manchester, and another from Plymouth, are darker, be- 
cause the muscovite nearly fails; one from Hooksett is lighter, because 
the biotite nearly fails. The Sunapee granites are in part light and in 
part dark. The beautiful Haverhill granite is very rich in quartz. A 
specimen from Effingham is very coarse and pseudo-porphyritic; but, 
with such minor variations, this granite, from all its localities, possesses 
the same distinctive microscopic structure and peculiarities. 
Biotite Granite. The granites in which biotite alone appears as the 
characteristic accessory are very numerous. Prominent among these are 
the Conway granites, which are characterized by their ready disintegra- 
tion. Any one who visits the region of Conway cannot but be struck 
with the immense amount of granitic sand and gravel which is there 
accumulated. Yards and streets are covered with the small fragments 
of disintegrated granites ; and not being rolled or water-worn, these bits 
preserve their angular forms. All around the country in the region of 
Conway numerous large blocks of granite are lying about, which one can 
