Sands have become “Sandstones”;
Gravel has become “Pudding-stone” (Conglomerate);
Mud and clay have become “Shale”;
Calcareous deposits have become “Limestone”;
Vegetable deposits have become “Coal.”
So we have sandy, clayey, limy, flinty, and corally rocks under long names respectively—Arenarious, Argillaceous, Calcareous, Silicious; and we may add Bitumenous and Ferruginous—Irony Rocks—to the list.
Speaking of sediments, it is curious to note the different colours of the Arve and the Rhone which meet near Geneva. The white sedimentary Arve can be traced for a long distance beside, not mixing with, the blue Rhone. The same effect can be traced where the latter river enters the Lake of Geneva. So the land is being perpetually carried away and deposited; and where water gains on land there is somewhere else always a corresponding elevation to compensate it. Thus places disappear, and the sea washes over the site, as on the Kentish Coast, where Earl Godwin’s land was inundated, and new land is reclaimed or is elevated from the sea to make up the balance.
“There rolls the deep, where grew the tree;
Oh, earth, what changes hast thou seen!
There, where the long street roars, has been
The stillness of the central sea.”
We have spoken of sedimentary and organic rocks. There is yet another kind called igneous, or fiery rocks—those upraised by volcanic action. Of the igneous rocks the crystalline have been evidently in a fused condition. Granite is an example; lava or basalt is the usual term for volcanic rock, and the basaltic caves of Staffa and the Giants’ Causeway bear testimony to the igneous or volcanic origin of the surroundings. The pillars and fantastic rocks of Ireland and Scotland which are so remarkable, are simply lava, which was erupted in a molten state, now cooled and contracted into blocks of curious regularity of form.
Fig. 644.—Trap rock (Staffa).