Page:The Habitat of the Eurypterida.djvu/133

 regions. In the northeastern portion of the Central Belt in the basin of the Gala Water, the various Birkhill zones are separated from each other by thick beds of grits, conglomerates, and greywackes. Even the graptolites show the effects of the great inpouring of fresh water, for not only are they rare, but those which are found are dwarfed as would be expected of a fauna dwelling in brackish water. Such features point beyond a doubt to the oscillatory conditions which prevailed along the shoreline and just so far as those conditions can be traced southward so far may we say the sea retreated in Llandovery and Tarannon time. It is only along the south central portion of the southern margin of the Central Belt that the highest Tarannon rocks are found; their continental origin is undoubted. They are unfossiliferous except for tracks and trails and they consist of grey, green, and red shales with bands of conglomerates one or two feet thick. All of these facts indicate a lithological replacement of marine by terrestrial deposits along a northwest-southeast line. The faunal replacement is equally striking. Along the northern border of the Tarannon belt the coarse deposits contain no fossils, but tracks and trails; when a few dark shale bands appear, they usually contain not good zonal fossils but a mixed Llandovery and Tarannon fauna. Monograptus exiguus is recognized as the lowest graptolite in the Tarannon and yet it frequently occurs with Rastrites maximus and Climacograptus normalis, the former of which is the zonal fossil for the uppermost Birkhill, and both of which are typical Llandovery forms. Towards the south, however, this interfingering and mingling of faunas is no longer noticeable and the Tarannon passes into the shaly, mudstone phase where zonal graptolites are well recognized, though in the passage to the upper Tarannon the mud facies is again replaced by conglomerates. The evidence supplied by the lithological and faunal characteristics, each taken independently, points conclusively to a replacing overlap and to the terrestrial origin of the Tarannon. The facts may be set forth in a generalized section.