236 THE OLD RED SANDSTONE. 
tains, and, among the rest, to a red granitic gneiss, which 
must heve been exposed over wide areas at the time of its 
deposition, and which, after the lapse of a period which ex- 
tended from at least the times of the Lower Old Red to those 
of the Upper Oolite, was again thrust upwards to the surface, 
to form the rectilinear chain of precipitous eminences to 
which the hills of Cromarty and of Nigg belong. This rock 
is now almost the sole representative, in the north of Scot- 
land, of the ancient rocks whence the materials of the -Old 
Red Sandstone were derived. It abounds in hematic iron ore, 
diffused as a component of the stone throughout the entire 
mass, and which also occurs in it in ponderous insulated 
blocks of great richness, and in thin, thread-like veins. 
When ground down, it forms a deep red pigment, undistin- 
guishable in tint from the prevailing color of the sandstone, 
and which leaves a stain so difficult to be effaced, that shep- 
herds employ it in some parts of the Highlands for marking 
their sheep. Every rawer fragment of the rock bears its 
heematic tinge; and were the whole ground by some mechan- 
ical process into sand, and again consolidated, the produce 
of the experiment would be undoubtedly a deep red sand- 
stone. In an upper member of the lower formation — that 
immediately over the ichthyolite beds— different materials 
seem to have been employed. A white, quartzy sand and a 
pale-colored clay form the chief ingredients; and though 
the ochry-tinted coloring matter be also iron, it is iron existing 
in a different condition, and in a more diluted form. ‘The 
oxide deposited by the chalybeate springs which pass through 
the lower members of the formation, would give to white 
sand a tinge exactly resembling the tint borne by this upper 
member. 
The passage of metals from lower to higher formations, 
