192 
BULLETIN OF THE NATURAL HISTORY SOCIETY. 
Stratigraphical Relations. — These may be most readily 
understood by the accompanying sectional view of the mountain 
and its immediate surroundings. As has been stated, nothing 
else is revealed in the mountain itself but compact crypto- 
crystalline diabase or doleryte, exposed both near its base and 
on its sides and summit. It thus forms a somewhat conical mass 
arising from the level of the river and presumably extending 
beneath it to an unknown depth and rising to a height of nearly 
three hundred feet. Upon either side the structure is quite 
different. Thus, quite near the base of the mountain on its 
northwestern side ? are beds of red conglomerate in nearly 
horizontal position, and which, if they do not underlie the whole 
mass of the mountain, as seems improbable, must have been 
penetrated by the volcanic rock in its efforts to reach the surface. 
This view is rendered almost certain by the fact that upon either 
side of the mountain similar red beds are exposed to within 
about sixty or seventy feet of the summits of the hills, being 
then capped by horizontal masses not of dolerite but of vesicular 
ash-rock and amygdaloid. Such open vesicular rocks are some- 
what of the nature of a slag, their cavities being the result of the 
expansion of gases under diminished pressure, and they are 
believed to have been formed at or near the surface, while those 
of a more solid crystalline character, such as constitute the 
mountain, were formed at lower levels and under greater 
resistance. Thus the mountain proper represents a volcanic 
neck or chimney, penetrating the red sediments from an un- 
known depths while the ash beds and amygdaloids are the 
lighter and more scoriaceous materials thrown off from the 
summit of the pipe, possibly under water, and spread over the 
surrounding deposits. These do not include the grey beds. At 
no point can the volcanic materials be found to rest upon the 
latter; but at no great distance to the north of the ravine the 
coarse grey beds of the Millstone Grit formation may be seen 
in a position which clearly indicates that they lie above the 
amygdaloids and ash-beds. 
