EXPLANATIONS OF iue SECTIONS AND PLATES

SECTION I.

Represents tlie Old Red System of Scotland from its upper beds ofYellow Quartzose Sandstone to its Great Conglomerate base. a. Quart-zose Yellow Sandstone, b. Impure concretionary limestone inclosingmasses of cliert. c. Red and variegated sandstones and conglomerate.These three deposits constitute an upper formation of the system charac-terized by its peculiar group of fossils. (See Chapter IX.) d. Deposit ofgray fissile sandstone, which constitutes the middle formation of the sys-tem, characterized also by its peculiar organic group. (See Chapter VIII.)e. Red and variegated sandstones, undistinguishable often in their mineralcharacter from the upper saudstones (c), but in general less gritty, and con-taining fewer pebbles. /. Bituminous schists, g. Coarse gritty sand-stone. h. Great Conglomerate. These four beds compose a lower forma-tion of the system, more strikingly marked by its peculiar organisms thaneven the other two. (See Chapters II., III., IV. , and V.) In the sectionthis lower formation is represented as we find it developed in Caithnessand 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 (/) is mineralogically different,—alternatingbeds of sandstone and clay, these last inclosing limestone nodules, takingthe place of the bituminous schists.

SECTION II.

The Old Red System of England and "Wales , as given in the generalSection of Mr Murchison, with the Silurian rocks beneath and the car-boniferous limestone above, i. The point in the geological scale at whichrertebrated existences first appear. The three Old Red Sandstone forma-tions of this section correspond in their characteristic fossils with those ofScotland , but the proportions in which they are developed are widely dif-ferent. The tilestones seem a comparatively narrow stripe in the system