The Classifying Principle and its Uses.--Three Groups of Ichthyolites among the Organisms of the Lower Old Red Sandstone.--Peculiarities of the Third Group.--Its Varieties.--Description of the _Cheiracanthus_.--Of two unnamed Fossils of the same Order.--Microscopic Beauty of these ancient Fish.--Various Styles of Ornament which obtain among them.--The Molluscs of the Formation.--Remarkable chiefly for the Union of modern with ancient Forms which they exhibit.--Its Vegetables.--Importance and Interest of the Record which it furnishes, 79-94 CHAPTER VI. The Lines of the Geographer rarely right Lines.--These last, however, always worth looking at when they occur.--Striking Instance in the Line of the Great Caledonian Valley.--Indicative of the Direction in which the Volcanic Agencies have operated.--Sections of the Old Red Sandstone furnished by the granitic Eminences of the Line.--Illustration.--Lias of the Moray Frith.--Surmisings regarding its original Extent.--These lead to an exploratory Ramble.--Narrative.--Phenomena exhibited in the Course of half an Hour's Walk.--The little Bay.--Its Strata and their Organisms, 95-108 CHAPTER VII. Further Discoveries of the Ichthyolite Beds.--Found in one Locality under a Bed of Peat.--Discovered in another beneath an ancient Burying-ground.--In a third underlying the Lias Formation.--In a fourth overtopped by a still older Sandstone Deposit.--Difficulties in ascertaining the true Place of a newly-discovered Formation.--Caution against drawing too hasty Inferences from the mere Circumstance of Neighborhood.--The Writer receives his first Assistance from without.--_Geological Appendix_ of the Messrs. Anderson, of Inverness.--Further Assistance from the Researches of Agassiz.--Suggestion.--Dr. John Malcolmson.--His extensive Discoveries in Moray.--He submits to Agassiz a Drawing of the _Pterichthys_.--Place of the Ichthyolites in the Scale at length determined.--Two distinct Platforms of Being in the Formation to which they belong, 109-124 CHAPTER VIII. Upper Formations of the Old Red Sandstone.--Room enough, for each and to spare.--Middle, or Cornstone Formation.--The Cephalaspis its most characteristic Organism.--Description.--The Den of Balruddery richer in the Fossils of this middle Formation than any other Locality yet discovered.--Various Contemporaries of the _Cephalaspis._--Vegetable Impressions.--Gigantic Crustacean.--_Seraphim._--Ichthyodorulites.--Sketch of the Geology of Forfarshire.--Its older Deposits of the Cornstone Formation.--The Quarries of Carmylie.--Their Vegetable and Animal Remains.--The Upper Formation.--Wide Extent of the Fauna and Flora of the earlier Formations.--Probable Cause, 125-150