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Most of the species of this genus are supposed to have been either river-fish, or inhabitants of the sea at the mouth of estuaries. Unio Valdensis, Mant. Underside of slab of sandstone about one yard in diameter. At different heights in the Hastings Sands, we find again and again slabs of sandstone with a strong ripple-mark, and between these slabs beds of clay many yards thick.
This resemblance is by no means confined to the existence in both occasionally of a laminated structure, but extends to every kind of arrangement which is compatible with the absence of fossils, and of sand, pebbles, ripple-mark, and other characters which the metamorphic theory supposes to have been obliterated by Plutonic action.
I shall accordingly proceed to treat of each family of rocks; first, in reference to those characters which are not chronological, and then in particular relation to the several periods when they were formed. Mineral Composition of Strata. Siliceous Rocks. Argillaceous. Calcareous. Gypsum. Forms of Stratification. Original Horizontality. Thinning out. Diagonal Arrangement. Ripple-mark.
Occasionally park-like views occurred, which, where diversified by the rocky valleys, resemble much the noble scenery of the Forest of Dean on the borders of Wales; the Mahowa especially representing the oak, with its spreading and often gnarled branches. Many of the exposed slabs of sandstone are beautifully waved on the surface with the ripple-mark impression.
In cases where the strata are curved, the planes of cleavage are still perfectly parallel. The true bedding is there indicated by a number of parallel stripes, some of a lighter and some of a darker colour than the general mass. Such stripes are found to be parallel to the true planes of stratification, wherever these are manifested by ripple-mark or by beds containing peculiar organic remains.
Similar undulating ridges and furrows may also be sometimes seen on the surface of drift snow and blown sand. The ripple-mark is usually an indication of a sea-beach, or of water from six to ten feet deep, for the agitation caused by waves even during storms extends to a very slight depth.
Thus the views previously entertained regarding the invertebrate character of the fauna of the Silurian epoch, have ultimately remained unaffected, so far as these Potsdam slabs are concerned. Slabs of sandstone and shale often retain what is called the ripple-mark that is, the corrugation of surface produced by the gentle agitation of shallow water over sand or mud.
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