114 CONTRIBUTIONS TO THE GEOLOGY OF MAINE. [bull. 165. 
CASTLE HILL ANDESITES. 
Castle Hill is the local name for the northern end of the high, nar- 
row ridge extending north-south across the township of that name. 
While not so conspicuous a feature as Haystack Mountain, at the 
southern end of the same ridge, it forms the most considerable promi- 
nence on the immediate bank of the Aroostook River, and as all the 
early travel lay along this stream, the ridge was an important land- 
mark to the first settlers. There is no common local usage as to the 
limits of Castle Hill, and in this report the term will be applied to the 
masses of andesite and volcanic elastics which lie between Aroostook 
River and the State road from Ashland to Presque Isle. It covers an 
area 24 miles long and varies in width from one-half to three-fourths 
of a mile. It is partly in Castle Hill Township and partly in Wade 
Plantation. The wagon road crosses the hill at the southern end, 
where it rises little higher than the surrounding plain. The east side 
has a gentle slope, and, being cut up into separate low knobs by small 
streams, the ridge effect is not apparent. The west side is formed by 
Welts Brook and tLe Aroostook River, which at this point is forced 
by it to take the abrupt backward turn so noticeable on the map. 
Calcareous and arenaceous slates are exposed in the bed of the river, 
while a short distance back steep slopes and cliffs of lava and ash 
rise to a height of several hundred feet. The hill is densely wooded 
and in places swampy, except at the southern end and along the east 
side. At these points the bare rocks are occasionally exposed and 
present great variation in character. In one place heavy ledges of 
gray andesite are exposed, particularly on the knobs occupying the 
northwest and southeast corners of lot 31. In the woods east of the 
mouth of Welts Brook is an outcrop of black siliciriecl tuff between 
slates. On the southeast corner of the hill are loose ash beds contain- 
ing fossils, coarse and fine volcanic breccias, and pumiceous lava in 
quite fresh condition. Where the old lavas have been planed off by 
glaciers and have been protected from weathering the outlines of 
bombs and pillows are plainly revealed. When weathered these bombs 
are loosened and drop out as oval or egg-shaped bodies with amygdal- 
oidal surface and denser interior, and lie about thickly strewing the 
fields. In one place there is a cisternlike depression in the solid andes- 
ite some 10 feet deep and 30 feet in diameter, while close about it are 
piled a great number of very vesicular bombs and much glassy and 
brecciated ash. Its appearance suggests a small blowhole made by a 
single explosion. The striking fact about all the volcanic accumula- 
tions in the Castle Hill region is their freshness and their umistakable 
character. 
DIABASE. 
Typical diabases have not been found at all in northern Maine, and 
rocks of diabasic habit and near enough in chemical composition to be 
