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 sandstones, 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. III. IV. and V.) In the 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 northern localities, at least one of the deposits, f, is mineralogically different--alternating beds of sandstone and clay, these last enclosing limestone nodules, taking the place of the bituminous schists.