In the following chapters I shall confine my observations chiefly to this system and its organisms. To none of the others, perhaps, excepting the Lias of the north of Scotland, have I devoted an equal degree of attention; nor is there a formation among them which, up to the present time, has remained so much a _terra incognita_ to the geologist. The space on both sides has been carefully explored to its upper and lower boundary; the space between has been suffered to remain well nigh a chasm. Should my facts regarding it--facts constituting the slow gatherings of years--serve as stepping-stones laid across, until such time as geologists of greater skill, and more extended research, shall have bridged over the gap, I shall have completed half my design. Should the working-man be encouraged by my modicum of success to improve his opportunities of observation, I shall have accomplished the whole of it. It cannot be too extensively known, that nature is vast and knowledge limited; and that no individual, however humble in place or acquirement, need despair of adding to the general fund. CHAPTER II. 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.