GREGORY.] DIABASE. 115 
classed as such are represented only by few outcrops, each of small 
extent. The most important exposure in the region is at Aroostook 
Falls, just over the international boundary line, in New Brunswick. 
Two small dikes of rock like that at Aroostook Falls were found crossing 
the east-west road about three-quarters of a mile northeast of Fair- 
mount Station on the Bangor and Aroostook Railroad. The width of 
the dikes and their contact with the calcareous slates were not made out. 
Other diabase dikes of small extent are found at the southwest base 
of Mars Hill. Besides the above outcrops, which will be further 
described, rocks of diabasic character were noticed along the railroad 
2 miles west of New Limerick, and a good example of a sheared 
diabasic rock was observed in a cut at milepost 140, a little south of 
Weeksboro Station on the Ashland branch of the Bangor and Aroos- 
took Railroad. 
AROOSTOOK FALLS DIABASE. 
As noticed by preA^ious writers, 1 this diabase occurs in a number of 
dikes cutting the blue limestone of the region, which at this place is 
closely folded and faulted along planes extending in more than one 
direction. The dikes are most abundant at the very foot of the falls, 
or series of rapids, where they have a uniform appearance and vary 
in size from mere dikelets 2 inches or more in width to a massive 
faulted and jointed dike 30 feet or more in width, which has affected 
the adjoining rocks in structure and texture. Most of the dikes 
trend N. 25° W., and dip W. < 60 =b, which makes them run at about 
right angles to the general strike of the sedimentaries. The dikes in 
the rapids above the falls form a series of abutments, causing narrows 
where they cross the stream, but they are so easily eroded that in 
other places they project little, if any, above the limestone which they 
cut, and if the present channel had not been occupied by the stream 
in recent geologic times, the dikes would be an inconspicuous feature 
of the river. As it is, the dikes can not be traced beyond the present 
valley. 
MARS HILL DIABASE. 
This diabase occurs in two dikes of different types situated close 
together on the southwest flank of the mountain. Walking up the hill 
from the McPherson house in the direction" of the bare ledge on the 
southern summit, one passes over calcareous slates and sandstones and 
comes to a dense blue-black rock, which appears to be a diabasic dike 
in direct contact with the slate conglomerate. As one climbs higher 
and over thick beds of arkose, another outcrop of igneous rock is seen 
at the base of a long talus slope. This is a second diabase dike, or 
perhaps a flow, of unknown width and extent, which attracts the atten- 
1 Jackson, Second Ann. Rept. Geol. Public Lands, 1838, p. 45. Hitchcock, Agriculture and Geology 
of Maine, 1861, p. 215. Bailey, Ann. Kept. Geol. Nat. Hist. Survey Canada, 1885, Pt. G., p. 22. 
