Gregory.] MARS HILL Dr ABASE. 177 
I. Diabase from Aroostook Falls, New Brunswick. Analysis by W. K. Hillebrand, United States 
Geological Survey. 
II. Diabase or chloritic dolerite from Lake Saltonstall, Connecticut. Analysis byG. W. Bawes, Trap 
rocks of the Connecticut Valley: Am. Jour. Sci., 3d series, Vol. IX, 1875, p. 190. 
III. Diabase from Reeds Gap quarry, Middlcfield, Connecticut. Analysis by Dr. J. II. Pratt, of Shei 
field Scientific School. (Not published.) 
IV. Diabase (dolerite) from West Rock, New Haven, Connecticut. Analysis by <;. W. Bawes, Trap 
rocks of the Connecticut Valley: Am. Jour. Sci., 3d series, Vol. IX, 1875, p. 186. 
V. Essexite (olivinegabbro-diabase). Dignacs. Kirchspicl Gran. Norway. Analysis by Schnulrk. 
Brogger, Quart Jour. Geol. Soc. London, Vol. L, 1894, p. 19. 
Discussion of analysis. — The above analyses show that in composition 
the Maine rock is very near those diabases of the Lower Connecticut 
Valley which have been mapped as extrusives, while it is somewhat 
lower in lime and higher in soda than the specimen from West Rock 
(IV), which is typical for the intrusive sheets of the region. Analysis 
V is given to show the relation to the essexites and rocks of similar 
character. Comparison with many analyses other than those given in 
the table suggests the idea that the Aroostook Falls dike is a transition 
between a diabase of normal type and rocks of the theralite-essexite 
magma. 
MARS HILL DIABASE. 
The two dikes found at this locality differ widely in appearance and 
type of structure. The easternmost dike closely resembles the rock 
described from Aroostook Falls and is considered as normal for this 
region. The other dike is exceedingly basic in composition and is 
microscopical^ characterized by the presence of a glassy base. 
NORMAL TYPE. 
The normal type of diabasic dikes forms the easternmost occurrence 
at Mars Hill. Owing to the presence of pyrite and a few feldspars 
larger than the microlites of the grounclmass, the rock appears slightly 
porphyritic to the unaided eye, and on much weathered surfaces it 
presents a bleached, slaggy appearance. Securely cemented within 
the diabase are fragments of a blood-red jasper, which vary in size 
from mere specks to blocks several feet square. These jasper inclusions 
are never rounded, but present straight, clear-cut edges against the 
diabase and show clearly the marks of stratification. It is also 
noticeable that the center of the jasper blocks is a dull brownish-red, 
amorphous-looking, baked slate, while at the contact with the igneous 
rock it becomes blood red and has a shiny crystalline appearance. 
These jasperlike fragments are considered siliceous iron-bearing slates, 
which have been broken off and included by the dike in its ascent, and 
which in consequence have been metamorphosed. 
Microscopic description. — Under the microscope feldspar and augite 
appear as the components. The few small phenocrystic feldspars are 
too badly decomposed to be determined, but if more basic than those 
constituting the groundmass. as is to be supposed, they are probably 
Bull. 165 12 
