344 iJi^ Americaji Geologist. December, loou 
A glance at the map will make clear the irregular distribu- 
tion of these dikes. They are all of contemporaneous origin, 
occupying probably original fissures in the Monhegan mass 
which are in no wise connected with subsequent zones of frac- 
ture. The dikes are themselves sheared and faulted to as great 
a degree as the rock in which they are intruded, and they are 
intersected by the granite dikes, already mentioned, which are 
consequently of more recent age. 
These dikes vary in thickness from a few inches to not 
more than three feet. They have but little persistency in 
strike, and only in rare instances could be traced for a consid- 
erable distance from the shore. The rock varies in color from 
dark steel gray to brownish or purplish gray, and in texture 
from aphaiiitic to medium granul:.r. The structure is pan- 
idiomorphic granular throughout; in contrast to that of the 
porphyritic diabase and camptonyte dikes, on th.e main-land.* 
The rock is remarkably fresh and exhibits none of the pro- 
nounced effects of erosion characteristic of the camp- 
tonj'tes. While showing, macroscopically, unappreciable dif- 
ferences, under the microscope, the mineral composition of 
these dikes is seen to lie between that of a hypersthene-gabbro 
on the one hand and gabbro-dioryte on the other — both ex- 
tremes being connected by types resembling hornblende-gab- 
bro. Adopting the classification of Chelius and Osann, the 
A. C. Lawson: op. cit.,,pp. 154-157, (F), and Amer. Geol., VII, 
1891, p. 153. 
U. S. Grant: The Geology of the Gabbro Lake Plate, Geol. and 
Nat. Hist. Survey of Minn., Vol. 4, 1896-98, p. 409. 
H. B. Patton: op. cit., p. 185. 
A. H. Elftman: loc. cit. 
H. W. Fairbanks: The Geol. of. Point Sal, Gal, Bull. Dept. Geol., 
Univ. of Gal., Vol. II, No. i, 1896, pp. 50-74. 
C. Chelius: Das Granitmassiv des Melibocus und seine Gang- 
gesteine. Notiz. d. Ver. fur Erdk., Darmstadt, 1892, IV. Folge, 13 
Heft, pp. 1-13, and 1894, IV Folge, 15 Heft, pp. 16-33. 
A. Osann: Ueber die dioritische Ganggesteine ini Odenwald. 
Mitth. d. Gross. Bad. Geol. Landesanstalt, 1892, II, p. 380. 
C. F. Kolderup: op. cit., p. 451. 
J. A. L. Henderson: Pet. and Geol. Investigations on certain 
Transvaal Norites etc.. London, 1898. (Dulau & Co., 27 Soho Sq.) 
p. 51. 
T. H. Holland: Basic Dikes in Norite from Southern India. 
Neues. Jahrb., Bd. II, Heft 3, 1898, pp. 441-83; and on Augite-Dior- 
ites with Micropegmatite. Q . J. G. Soc, 1897, pp. 403-417. 
*See E. C. E. Lord: On the Dikes in the Vicinity of Portland, Me. 
Am. Geol, Vol. XXII, Dec, 1898, p. 339- 
