Owing to the accumulation of thick masses of sediment, the Lower Carboniferous sea of the north of England appears to have been largely silted up, and although the organic deposits of the south are so thin that they did not render the sea shallow in that region, the general level of the Lower Carboniferous floor of the south was also uplifted, and actually converted into land, as the result of the upward movement which took place in Devonshire and tracts of France; and owing to silting up in the north, and elevation in the south, a general plane surface was produced over very extensive areas, not only in Britain but upon the Continent, upon which the peculiar deposits and accumulations of Upper Carboniferous times were laid down, sometimes in shallow water, sometimes upon the land, and often under conditions which cannot at present be determined with accuracy. That the deposits of the Millstone Grit and Coal Measure epochs were to a large extent laid down in water is admitted by all, and in the case of many of the deposits of the Millstone Grit, and some thin deposits of the Coal Measures, it is equally clear that the water area was part of an expanse of ocean, for we find marine fossils, as corals, crinoids, and cephalopods, in these beds. Associated with them in the Coal Measures are other beds in which the ordinary Carboniferous genera of marine invertebrates are absent, and their place is taken by shells which bear much resemblance to the modern fresh-water mussel, and it has been maintained with good reason that as the ordinary marine forms are rarely or never mixed with those resembling recent fresh-water shells, the latter are truly fresh-water[89]. If this be so, many of the mechanically formed sediments of the Coal Measures were of fresh-water origin, laid down in shallow lagoon-like expanses, probably shut off from the main ocean by a narrow portion of intervening land, which was occasionally destroyed, thus permitting incursions of salt-water when some of the ordinary marine invertebrates of the period obtained a temporary footing in the area. [Footnote 89: For further information upon this subject the student should consult the Introduction to a Monograph on _Carbonicola_, _Anthracomya_ and _Naiadites_ (the shells in question) by Dr Wheelton Hind, being one of the Monographs of the Palæontographical Society.]