OF THE HILL OF SEETABULDEE. 
299 
A.t the junction of these rocks, the passage is sometimes so 
gradual, as to give the intermediate rock an indeterminate 
character, partaking of the nature of both. At others, it is 
abrupt ; yet, notwithstanding the abruptness of the change, 
the vei'tical and horizontal fissures are prolonged into each 
other, and cross the line of junction. I shall not here enter 
into a more minute detail of these appearances, but shajl 
content myself with observing, that the most satisfactory 
explanation of these phenomena is derived from that theory 
which ascribes to the trap-rocks an igneous origin, under 
pressure of a great body of water. 
The semi-columnar basalt, forming the greater part of 
the hill, is very porous, containing numerous amygdaloidal 
cavities, which are for the most part merely lined with a 
peculiar mineral, which I presume to name Conchoidal 
Augite. Sometimes, however, they are nearly filled with it, 
or with calcedony, semi-opal, or carbonate of lime; the 
calcedony being usually covered with a coating of green 
earth. The rock itself is composed of hornblende and fel- 
spar, with the augite so profusely disseminated, that it 
ought to be considered, in some cases, as a constituent of 
the rock. The rock is fusible, and is of considerable spe- 
cific gravity, notwithstanding its porosity. The vertical 
and horizontal fissures are not always straight, but are at 
times waved ; they are also sometimes lined with an infil- 
tration of calcedony coated with clay and chlorite. 
This rock is succeeded by an indurated clay or wacke, 
which, at its junction with the basalt, frequently partakes 
of its semi-columnar structure, the vertical fissures being 
prolonged into the wacke, and the schistose structure of the 
latter extending into the basalt. These changes may be 
very favourably seen in the fosse which surrounds the for- 
tification. 
