inch or two in thickness, gradually thins away at each extremity, sometimes to a hair's breadth, at other times shooting of like a spreading root of delicate fibres. Very often a slender thread cuts across a thick vein, throwing it several inches out of its regular course, affording a miniature representation of the disturbances in the great veins of mining districts. These veins do not penetrate the slate either above or below the stratum of limestone in which they are contained: but there are other veins of calcareous spar which cut through the strata, generally occasioning a dislocation, and in many instances I observed the substance of the vein penetrating the adjoining strata in minute ramifications. It will be difficult to reconcile these appearances to any theory of veins that has yet been proposed; like other instances, which every country affords, they tell us how little we yet know of the laws by which the materials of our globe have been brought together into their present arrangement.
§ 38. The slate clay that is interposed between the strata of limestone assumes different appearances. In some places it is grey, in others brown, and in others nearly black: it is frequently bituminous, having, when broken, a strong fetid smell. The strata are divisible into laminæ as thin as common pasteboard, splitting with the greatest facility, and where it is washed by the sea it is very soft and friable. It appears to contain the same fossils as the limestone, and it is in this that the flattened ammonites are found; those in the limestone preserving their natural form, as far as my observations went. It frequently contains imbedded masses of a limestone identical with that of the regular strata, of a lenticular shape similar in form to the masses of clay-ironstone found in the clay of the coal formations. These are frequently uniform in structure throughout, sometimes they contain slender veins of calcareous
