Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/378

 sent, owing to the uncongenial quality of the water, many of the Black-sea shells are strangely distorted, as shown by Edward Forbes.

Or if we take the Caspian alone, as it now stands, as an example, we have a salt inland sea which, according to accepted views, was once united to the North Sea and, possibly, at the same time to the Black Sea, as shown by the Aralo-Caspian deposits, at a time when the Bosphorus was still closed. Changes in physical geography have taken place of such a nature that the Caspian is now disunited from the ocean, while its waters are still inhabited by a poor and dwarfed marine molluscan fauna, and by seals. If by increase of rainfall the Caspian became freshened, evaporation not being equal to supply, it would by and by, after reaching the point of overflow, be converted into a great freshwater lake, larger in extent than the whole area now occupied by the Old Bed Sandstone of Europe *. It is even conceivable that the great area of inland drainage of Central Asia, now holding many salt lakes, might in the same manner be so changed that all its lakes would become fresh and widened in extent, thus occupying areas many times as large in extent as all the known European Old Red Sandstone. Under these circumstances, in the Caspian area we should have a passage more or less gradual from marine to freshwater conditions, such as I conceive to have marked the advent of the Old Red Sandstone. When the whole area was fairly isolated from the sea, the sediments might by degrees get into a condition to be coloured red in the manner previously mentioned. We have a case in point in an old inland sheet of water, as shown by the red marls of the Miocene lakes of Central France. In certain of the strata of the Old Bed Sandstone, especially in the upper beds, the colour has been here and there discharged in irregular patches, probably through the reducing action of organic matter, and the percolation of water containing carbonic acid.

Mr. Jukes divided the Old Red Sandstone of Ireland into two portions. The lower series is conformable with and adheres to the Upper Silurian strata. The upper series lies quite unconformably on the lower, and adheres to and is conformable with the Carboniferous strata. In Wales and the adjoining counties no such unconformity has been clearly made out and mapped, though Sir Henry De la Beche pointed out that such a division may exist, as shown by the overlap of the upper strata across the lower, proceeding westward from Breconshire into Carmarthenshire and Pembrokeshire. However this may be, the thickness of the Old Red Sandstone is often very great ; and this to some minds, taken perhaps in conjunction with unconformity, may present a difficulty to the acceptance of my view. In South Wales and Herefordshire the formation is from 2000 to 7000 or 8000 feet thick, as determined by my measurements carefully levelled in 1843. But on consideration these circumstances do not appear to present any real difficulty. If the great hollow in which the Dead Sea lies were gradually to get filled with fresh water and silted up, 1300 feet would be added above the level of the present surface, without taking into account the depth


 * Exclusive of marine Devonian rocks.