EXPLANATIONS OF THE SECTIONS AND PLATES. 
SECTION I. 
ReprEsENTs the Old Red System of Scotland from its upper beds 
of Yellow Quartzose Sandstone to its Great Conglomerate base. 
a. Quartzose Yellow Sandstone. %. Impure concretionary limestone 
enclosing masses of chert. ec. Red and variegated sandstones and 
conglomerate. These three deposits constitute an upper formation 
of the system, characterized by its peculiar group of fossils. (See 
Chapter IX.) d. Deposit of gray fissile sandstone which constitutes 
the middle formation of the system, characterized also by its peculiar 
organic group. (See Chapter VIII.) e. Red and variegated sand- 
stones, undistinguishable often in their mineral character from the 
upper sandstones, c, but in general less gritty, and containing fewer 
pebbles. f. Bituminous schists. g. Coarse gritty sandstone. h. Great 
Conglomerate. These four beds compose a lower formation of the 
system, more strikingly marked by its peculiar organisms than even 
the other two. (See Chapters II. WI.IV.and V.) Inthe section this 
lower formation is represented as we find .it developed in Caithness 
and Orkney. In fig. 5 it is represented as developed in Cromarty, 
where, though the fossils are identical with those of the more north- 
ern localities, at least one of the deposits, f, is mineralogically differ- 
(xxi) 
