The Old Red Sandstone.--Till very lately its Existence as a distinct Formation disputed.--Still little known.--Its great Importance in the Geological Scale.--Illustration.--The North of Scotland girdled by an immense Belt of Old Red Sandstone.--Line of the Girdle along the Coast.--Marks of vast Denudation.--Its Extent partially indicated by Hills on the western Coast of Ross-shire.--The System of great Depth in the North of Scotland.--Difficulties in the Way of estimating the Thickness of Deposits.--Peculiar Formation of Hill.--Illustrated by Ben Nevis.--Caution to the Geological Critic.--Lower Old Red Sandstone immensely developed in Caithness.--Sketch of the Geology of that County.--Its strange Group of Fossils.--their present Place of Sepulture.--Their ancient Habitat.--Agassiz.--Amazing Progress of Fossil Ichthyology during the last few Years.--Its Nomenclature.--Learned Names repel unlearned Readers.--Not a great deal in them, 15-34 CHAPTER III. Lamarck's Theory of Progression illustrated.--Class of Facts which give Color to it.--The Credulity of _Unbelief_.--M. Maillet and his Fish-birds.--Gradation not Progress.--Geological Argument.--The Present incomplete without the Past.--Intermediate Links of Creation.--Organisms of the Lower Old Red Sandstone.--The _Pterichthys_.--Its first Discovery.--Mr. Murchison's Decision regarding it.--Confirmed by that of Agassiz.--Description.--The several Varieties of the Fossil yet discovered.--Evidence of violent Death in the Attitudes in which they are found.--The Coccosteus of the Lower Old Red.--Description.--Gradations from Crustacea to Fishes.--Habits of the _Coccosteus_.--Scarcely any Conception too extravagant for Nature to realize, 35-54 CHAPTER IV. The Elfin-fish of Gawin Douglas.--The Fish of the Old Red Sandstone scarcely less curious.--Place which they occupied indicated in the present Creation by a mere Gap.--Fish divided into two great Series, the Osseous and Cartilaginous.--Their distinctive Peculiarities.--Geological Illustration of Dr. Johnson's shrewd Objection to the Theory of Soame Jenyns.--Proofs of the intermediate Character of the Ichthyolites of the Old lied Sandstone.--Appearances which first led the Writer to deem it intermediate.--Confirmation by Agassiz.--The _Osteolepis_.--Order to which, this Ichthyolite belonged.--Description.--_Dipterus._--_Diplopierus._-- _Cheirolepis._--_Glyptolepis_, 55-78 CHAPTER V.