Petrography of Mount Orford. — Dresser. 17 
obscured by the excessive alteration of its component miner- 
als, the primary constituents that remain being comparatively 
rare. In specimens from the summit the following minerals 
were distinguished: plagioclase feldspar, generally quite tur- 
bid from decomposition, though retaining its crystal outlines 
and occasionally plainly showing the distinctive multiple 
twinning ; aggregates of chlorite, epidote and a light green or 
colorless hornblende ( ?) which are taken to represent pri- 
mary pyroxene ; an abundance of leucoxene, indicative of the 
existence of primary iron ore and indicating its titaniferous 
charactei ; quartz, which is also apparently of secondary ori- 
gin. 
The arrangement of the feldspars in reference to aggre- 
gates of pyroxenic decomposition products, and to one an- 
other, is that characteristic of the structure of diabase, to which 
the composition indicated above also allies the rock. It is 
therefore an altered diabase closely resembling that known 
to occur at Owl's Head,* sixteen miles to the southward. 
But it has been r.aid that the rock is coarser in the central 
portions, while other variations also appear in it on more de- 
tailed examination. In these coarser parts such as are seen 
for the first one-third or half of the way up the mountain on 
the south side, the original structure was more granular than 
that of a diabase. The mineral olivine also appears, either 
enclosed in or associated with larger masses of fibrous horn- 
blende, while the other mineral constituents are essentially 
the same as in the preceding specimens. While a rock of this 
composition and structure would be a dioryte, yet as the 
hornblende, from its fibrous character, irregular outlines and 
the probable presence of a little pyroxene in one slide, is 
likely in a large measure uralitic, it would have been origin- 
ally a gabbro. It is accordingly best classed as a gabbro- 
dioryte , in the sense in which the term was used by Prof. G. 
H. Williamsf in connection with an apparently similar phase 
of the Baltimore gabbros to indicate not only the composition 
and structure of the rock, but also its origin. 
•"Notes on the Microscopic Structure of Some Rocks of the Quebec Group," 
by Dr. F. D. Adams, Rep. Geol. Survey of Canada. 1880-1-2-3, p. 13 A. 
^Bulletin, U. S. Geol. Survey, No. 28, p. 17, et aliter. The term is similarly 
used by F. D. Chestkr, "Gabbro and Associated Rocks of Delaware," Bulletin 
U. S. Geo!. Sur. No. 58, pp. 1.5-19. 
