160 
The American Geologist. 
March, 1897 
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Fro. 4. — Plan and Sections of the Great Vein of Peg-matite forming Osg-ood's 
Ledge, Acworth, N. H. One inch = 400 feet. 
We were surprised to find that the cavities occupied by the 
pegmatite are, on the whole, more crack-like (parallel sides, 
definite angles, and general linear character) and more plainly 
suggest a mechanical or disrupting force in the massive rocks 
than in the bedded and schistose rocks; and for many of the 
pegmatite masses in the schists the mechanical explanation is 
entirely inadmissible, the pegmatite having evidently made 
room for itself by dissolving and absorbing considerable vol- 
umes of schist — spaces of dissolution. We have a magnificent 
example in the great body of pegmatite at Ruggles Mica mine, 
in Grafton. ^ It has great breadth in the vertical schists, and 
the schists, without change of dip, completely cover the pegma- 
tite, the bedding planes ending downwards squarely against 
the approximately flat upper surface of the great boss of stock 
of pegmatite (Figure 7). Evidently, no theory of pegmatite 
can be regarded as adequate which fails to take account of 
both of these diverse causes of the cavities — rifting and solu- 
