THE OLD RED SANDSTONE. 23 
inferior rocks. The Old Red Sandstone of Morvheim, in 
Caithness, overlooks all the primary hills of the district, from 
an elevation of three thousand five hundred feet. 
The depth of the system, on both the eastern and western 
coasts of Scotland, is amazingly great — how great, I shall not 
venture tosay. There are no calculations more doubtful than 
those of the geologist. The hill just instanced (Morvheim) is 
apparently composed from top to bottom of what in Scotland 
forms the lowest member of the system —a coarse conglom- 
erate ; and yet [ have nowhere observed this inferior mem- 
ber, when I succeeded in finding a section of it directly ver- 
tical, more than a hundred yards in thickness —less than 
one tenth the height of the hill. It would be well nigh as 
unsafe to infer that the three thousand five hundred feet of 
altitude formed the real thickness of the conglomerate, as to 
infer that the thickness of the lead which covers the dome 
of St. Paul’s is equal to the height of the dome. It is always 
perilous to estimate the depth of a deposit by the height of a 
hill that seems externally composed of it, unless, indeed, like 
the pyramidal hills of Ross-shire, it be unequivocally a hill 
dug out by denudation, as the sculptor digs his eminences 
out of the mass. In most Of our hills, the upheaving agency 
has been actively at work, and the space within is occupied 
by an immense nucleus of inferior rock, around which the 
upper formation is wrapped like a caul, just as the vegetable 
mould or the diluvium wraps up this superior covering in 
turn. One of our best known Scottish mountains— the gi- 
gantic Ben Nevis—furnishes an admirable illustration of 
this latter construction of hill. It is composed of three zones 
or rings of rock, the one rising over and out of the other, 
like the cases of an opera-glass drawn out. The lower zone 
is composed of gneiss and mica-slate, the middle zone of 
