696 Dikes of the Hudson River Highlands. 
tioned, contains, however, magnetite inclusions, but in other respects 
shows no peculiarities worthy of note. The feldspar is in irregular 
masses, well twined, and contains not unfrequently as inclusions 
the hornblende and augite, as well as magnetite and apatite. By 
powdering the rock and carefully separating the feldspar by means 
of the Thoulet’s heavy solution the specific gravity was found to lie 
between 2.67 and 2.70, bringing it near Labradorite." The 
magnetite is in coarse, irregular masses, showing no indications of 
titanium. ‘The apatite prisms are also quite coarse. It follows, 
therefore, from the relations above set forth, that, according to the 
well-known general law, the magnetite and apatite have been 
first to form, and have then been taken up in the mass of the horn- 
blende, augite and biotite in their subsequent crystallization, while 
the feldspar has been last of all to form, as it includes all the 
others. In the accompanying figure (Fig. 2), taken froma micro- 
photograph, the structure of a typical specimen is shown. he 
actual field is 4.0 mm. in diameter. The different minerals, horn- 
blende, augite and plagioclase are well indicated by their characteristic 
cleavages. 
In the matter of classification these rocks are perhaps most 
nearly allied to those called by Rosenbusch Camptonites,” but 1 
the matter of structure it should be stated they differ widely from 
the typical Camptonites in the writer’s possession, for anything like 
a porphyritic structure is entirely lacking. Although differing 
from the typical and widely-known kersantites in the small 
` amount of biotite contained, they yet, with some described 
varieties,“ seem structurally related. A normal kersantite has 
been described from this region by Dr. Newberry," which seems to 
form a boss in the enclosing gneiss at Croton Point. 
1 Rosenbusch. Mikros-Phys. 2d Ed., vol. i., p. 535. 
2 Rosenbusch. Neues Jahrbuch f. Min., ete., 1882, II., P- 1-16. a 
ralist for March and April, 1888, p. 207, ete. It is much to be regretted 
that Professor Bayley did not translate in this connection the German 
word “gang” by our English equivalent “dike.” The Engli 
“vein”? is now so well understood in all the literature on ore dé 
to refer to a mineral body deposited from solution, that an intrusi 
igneous vein seems a misnomer. - 1884, 
‘Cf. Pohlmanns. 4th Group. Neues Jahrb, Beil. Band., III., 
ye or 
p. 67. 5 
5 The School of Mines Quarterly, vol. viii., p. 330, July, 1837. 
