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.

SECTION II.

The Old Red System of England and Wales, as given in the general Section of Mr. Murchison, with the Silurian Rocks beneath and the carboniferous limestone above. _i._ The point in the geological scale at which vertebrated existences first appear. The three Old Red Sandstone formations of this section correspond in their characteristic fossils with those of Scotland, but the proportions in which they are developed are widely different. The tilestones seem a comparatively narrow stripe in the system in England; the answering formation in Scotland, _e, f, g, h_, is of such enormous thickness, that it has been held by very superior geologists to contain three distinct formations--_e_, the New Red Sandstone, _f_, a representative of the Coal Measures, and _g, h_, the Old Red Sandstone.

SECTION III.

Interesting case of extensive denudation from existing causes on the northern shore of the Moray Frith. (See pages 197 and 198.) The figures and letters which mark the various beds correspond with those of fig. 5, and of the following section. The "fish-bed," No. 1, represents what the reader will find described in pp. 221-225 as the "platform of sudden death."

SECTION IV.

Illustration of a fault in the Burn of Eathie, Cromartyshire. (See pages 204 and 205.)

EXPLANATIONS OF THE PLATES.

Plate I.--Fig. 1, Restoration of upper side of the elongated species of _Pterichthys_ (_P. oblongus_,) referred to in page 47. Fig. 2, _Pterichthys Milleri_. Fig. 3, Part of tail of elongated species, showing portions of the original covering of rhomboidal scales. Fig. 4, Tubercles of _Pterichthys_ magnified.

Plate II.--Fig. 2, Restoration of under side of _Pterichthys oblongus_. Fig. 1, A second specimen of _Pterichthys Milleri_. Fig. 3, Portion of wing, natural size.

Plate III.--Fig. 1, _Coccosteus cuspidatus_. Fig. 2, Impression of inner surface of large dorsal plate. Fig. 3, Abdominal lozenge-shaped plate. Fig. 4, Portion of jaw, with teeth.

Plate IV.--Fig. 1, Restoration of _Osteolepis major_. Fig. 2, Scales from the upper part of the body magnified. Fig. 3, Large defensive scale which runs laterally along all the single fins. Fig. 4, Under side of scale, showing the attaching bar. Fig. 5, Enamelled and punctulated jaw of the creature. Fig. 6, Magnified portion of fin, showing the enamelled and punctulated rays.

Plate V.--Fig. 1, _Dipterus macrolepidotus_. This figure serves merely to show the place of the fins and the general outline of the ichthyolite. All the specimens the writer has hitherto examined fail to show the minuter details. Fig. 2, _Glyptolepis leptopterus_. Fig. 3, Single scale of the creature, showing its rustic style of ornament. Fig. 4, Scale with a nail-like attachment. Fig. 5, Under side of scale. Fig. 6, Magnified portion of fin. Fig. 7, Shells of the Old Red Sandstone.

Plate VI.--Fig. 1, _Cheirolepis Cummingiæ_. Fig. 2, Magnified scales. Fig. 3, Magnified portion of fin.

Plate VII.--Fig. 1, _Cheiracanthus microlepidotus_. Fig. 2, Magnified scales. Figs. 3, 4, 5, 6, 7, 8, Vegetable impressions of the Old Red Sandstone.

Plate VIII.--Fig. 1, _Diplacanthus longispinus_. Fig. 2, _Diplacanthus striatus_. Fig. 3, Magnified scales of fig. 1. Fig. 4, Spine of fig. 2, slightly magnified.

Plate IX.--Fig. 1, One of the tail flaps of the gigantic Crustacean of Forfarshire. Fig. 2, Reticulated markings of Carmylie.

Plate X.--Fig. 1, _Cephalaspis Lyellii_, copied from Lyell's _Elements of Geology_, Fig. 2, _Holoptychius nobilissimus_, copied on a greatly reduced scale from Murchison's _Silurian System_, Fig. 3, Scale of _Holoptychius_, natural size. Fig, 4, Tooth of ditto, also natural size. These last drawn from specimens in the collection of Mr. Patrick Duff, of Elgin.

DIRECTIONS TO THE BINDER.

Sheet of Sections to front Title-Page. Plate I. to front page 44 II. " " " 46 III. " " " 48 IV. " " " 66 V. " " " 72 VI. " " " 78 VII. " " " 82 VIII. " " " 84 IX. " " " 136 X. " " " 154

NEW WALKS IN AN OLD FIELD;

OR,

THE OLD RED SANDSTONE.