THE DOLEEITES OF FINGER MOUNTAIN. 
51 
Sandstone, of which not more than 100 feet appears. Above this there is a 
sheet of columnar dolerite 200 feet thick, which on the west side of the hill 
unites with another sheet of dolerite. These two sheets are separated by the 
wedge of sandstone already referred to in Chapter VI. The bedding-planes of 
this wedge are horizontal, and are made conspicuous by the intrusion of numerous 
thin sills of dolerite along them. The wedge tapers to the west, and at this end 
the columns of dolerite do not appear continuous throughout the cliff, but break 
and bend over to the west at a line which follows the inclination of the upper 
surface of the now absent wedge. Finger Mountain narrows eastward to a sharp 
spur (bj-ba). The upper sheet of dolerite ends at a scarp at the summit of B, 
while the lower continues some distance and is cut off by a structure-line, parallel 
to the line of transgression followed by the sill above the sandstone-wedge. This 
spur is capped by an outlier of the dolerite, which is separated by sandstone from 
a lower sill of transgressive dolerite 200 feet thick. 
Specimens were collected both from the last sheet (692) and from a dyke 
(691) cutting across the bedding-planes between this and the one above. Dykes 
and sills are numerous at this rather disturbed locality. A sill of dolerite 30 feet 
thick extends for a hundred yards along a bedding-plane, then terminates suddenly 
with a vertical end. Another sill 10 feet thick runs along a bedding-plane for 
50 yards, breaks steeply downwards for 50 feet, and then, forcing its way along 
a bedding-plane for 100 yards, finally thins out and disappears. A third sill, 
2 feet thick, extends 50 yards along a horizontal bedding-plane, but gradually 
decreases in thickness and ends as a wedge. 
The specimens (695, 696 ; see p. 138) were collected from the base of x, from 
a sheet of dolerite below 200 feet of sandstone. Here also the dolerite occurs 
in sheets wdiich alternate with the layers of sandstone, and dykes and thin 
sills are numerous. 
Knob Head Mountain (B g ) (Fig. 26). 
At a height of 3000 feet above the sea, and 30 miles inland, close under the 
foot of Knob Head Mountain, which is over 8000 feet in altitude, there is a cliff, 
100 yards in length and 200 feet high, composed of columnar dolerite. Above the cliff 
the hillside slopes up more gently, and is covered with drift-blocks of granite and 
dolerite ; the covering is broken only by this exposure of rock near its base. The outcrop 
shows columns 12 feet in diameter, and from 20 to 200 feet in height. There are 
occasional horizontal cross-joints, but cup-and-ball structure is not developed. The 
Beacon Sandstone appeared to rest upon this dolerite-mass (661) ; it forms the 
main mass of the mountain, but the junction of the two could not be found. The 
hill B (; , three or four miles to the west, consists mainly of sandstone, but is 
riddled by dykes which form a network on its surface. On its west side there is 
h 2 
