694 Dikes of the Hudson River Highlands. 
Under the microscope it is seen to consist chiefly of innumerable 
small but well-developed hornblende crystals, having sharply de- 
fined prismatic and pinacoidal faces. They vary from 0.02 mm. 
to 0.05 mm. in width, and are about five times as long. The 
accompanying drawing (Fig. 1), from a micro-photograph, illus- 
trates the structure. The actual field is one millimetre in diameter. 
With the hornblende is some plagioclase not very well developed, 
as the rock has more or less of a porphyritic facies. Magnetite is 
sparsely scattered throngh it in very small grains. Slides of this rock 
are well nigh indistiaguishable from those of the diorite 
described by Hawes from Campton,! N. H., and later by Professor 
Harrington from Montreal, resembling? less closely the similar diorite 
described by the writer from the Forest of Dean mine? Mr. L. 
M. Dennis, Instructor in Chemistry in Cornell University, has 
made the following analysis, No. 1 in duplicate. No. 2 is of the 
Campton dike by Hawes, No. 3 of the Montreal dike by Professor 
Harrington, No. 4 of the Forest of Dean by the writer. 
I II. III 
BES 
Si O, 144.85 | 44. 41.94 | 40.95 
Al, O; 117.20 | 17.281 36 | 16.45 
Fe, 0, | 11.20 |i 3.27 | 13.47 
PeO cc aa a ATR VE 
Mn O Í trace | trace 25 
Ti O, | 6.578 | 6. 4.15 3.3 
Ca O Í 7.52 7.54 9.47 | 10.53 
Mg O 1 5.02 4.946 5.01 6. 
K,O } 2.992 | 2.621 19 1. 
Na, O Í 1.390 | 1.611 5.15 | 4.00 
P, O, I 383 ber Po o. : 
G O, SOn es C478 oa 
Loss on ignition | 2.387 | 2.491 3.29 | 3.84 
Totals 199.520 | 99.585 | 100.44 § 100.63 
The discovery of this rock is interesting as showing the further 
occurrence of dikes of this character, On the basis of Hawes’ 
description, Rosenbusch has made a separate type of dike rocks, 
and called them Camptonites. Various outcrops of so-called se 
dikes are recorded by the New York Survey in other parts 
* Am. Jour. Sei. III., vol. xvii., p. 147. 
? Geol. Survey Canada, 1877-78, A 439. 
3? Am. Jour. III., vol. xxxv. ; p. 351. 
t Rosenbusch, Mikros-Physiographie, 2d Edition, vol. i., p- 333. 
