162 TRANSACTIONS OF THE [maR. 19, 
es Hitchcock. If the parent ledge is ever discovered, it will 
readily be recognized from the peculiar character of the rock. 
The boulder donated to the American Museum of Natural 
History by Dr. Hubbard was originally an irregular ovoid, 
about two and a half feet in each of two directions and rather 
less than two feet in the third. It weighed more than 1,200 
pounds. It has been broken longitudinally so as to expose a 
fresh surface to view. The most conspicuous feature of the 
rock is the numerous rounded masses of granular olivine scat- 
tered through it, more than fifty of which, one inch in diameter 
and upwards, were counted on the freshly fractured surface 
alone, to say nothing of the numerous small nodules. The 
largest masses measured were 53 inches by 64 inches, 32 inches 
in diameter, 44 inches by 35 inches and 41 inches by. 21 inches 
in dimensions. In appearance these nodules somewhat resemble 
the bombs of olivine found in several areas of basaltic eruption, 
notably the Eifel of Rhenish Prussia. The Thetford olivine 
sent to the Sheffield Scientific School was analyzed there by E. 
A. Manice, with the following result : * 
to) LO eee AOr eam ctsUN TIC! 
Re LOcd hea eee 
Max Ood A.95) 00L28 
Total  . 100.39 
This corresponds almost exactly to the theoretic composition 
_of an olivine (chrysolite) in which the Mg is to the Fe in the 
ratio of 10 tol. The granular masses, while they contain many 
particles of apparently unaltered material, show the usual ten- 
dency toward alteration, and the small nodules have for the 
most part already gone over into serpentine. 
The other striking constituent of the rock is the large 
rounded crystalline masses of grayish green, glassy pyroxene. 
Some of the largest on these measured 3 inches by 2} inches, 3 
inches by 23 inches, 24 inches by 14 inches and 13 by ? inches 
on the sections exposed. They may be gotten out whole or 
nearly whole from the matrix, and, when isolated, they present 
the appearance of waterworn pebbles with pitted surfaces. 
Each seems to be a single crystal and they may best be ex- 
plained as the remains of great crystals of augite, which have 
been rounded by extensive magmatic re-absorption. Under the 
microscope this pyroxene is seen to be an augite with its angle 
of extinction not less than 44°. It is very pale green in the 
thin section and consists of broad areas of bright, fresh material 
*Am, Jour. Sci., II., xxxi., 359, 1861. Also: Syst. Mineralogy, Dana, 5th Ed., 
p. 257. 
