THE OLD RED SANDSTONE. 99 
which have been broken into pebbles for the construction of 
the one, have been ground into sand for the composition of 
the other. Directly over them, the middle portion of the pyr- 
amid is occupied by an enormous deposit of dark-colored 
bituminous schist, slightly micaceous, calcareous, or semi-cal- 
careous, — here and there interlaced with veins of carbonate 
of lime,--here and there compact and highly siliceous, — 
and bearing in many places a mineralogical character diffi- 
cult to be distinguished from that at one time deemed peculiar 
to the harder grauwacke schists. The Caithness flagstones, 
so extensively employed in paving the footways of our larger 
towns, are furnished by this immense middle tier or belt, and 
represent its general appearance. From its lowest to its 
highest beds it is charged with fossil fish and obscure vegeta- 
ble impressions ; and we find it represented in the Cromarty 
section by alternating bands of sandstones, stratified clays, 
and bituminous and nodular limestones, which form altogether 
a bed three hundred and fifty-five fect in thickness ; nor does 
this bed lack its organisms, animal and vegetable, generically 
identical with those of Caithness. The apex of the pyramid 
is formed of red mouldering sandstones and mottled marls, 
which exhibit their uppermost strata high over the eddies of 
the Pentland Frith, in the huge precipices of Dunnet-head, 
and which are partially represented in the Cromarty section 
by an unfossiliferous sandstone bed of unascertained thick- 
ness ; but which can be traced for about eighty feet from the 
upper limestones and stratified clays of the middle member, 
until lost in overlying beds of sand and shingle. 
I am particular, at the risk, lam afraid, of being tedious, 
in thus describing the Geology of this northern county, and 
of the Cromarty section, which represents and elucidates it 
They illustrate more than the formations of two insulated 
3 * 
