Page:The Habitat of the Eurypterida.djvu/131

 of this succession will be spoken of presently. Continuing at right angles to the strike, there is found about 2 miles northwest of Hartfell on the Cow Linn, the last outcrop of the Monograptus gregarius zone. Only 3$$\tfrac{1}{2}$$ miles northwest of this locality, near the junction of the Fruid water with the River Tweed the gregarius zone is no longer to be found, the highest of the Birkhill beds being the Diplograptus vesiculosus zone (a2) which is the second in the Lower Birkhill series. This zone is immediately followed by the Tarannon grits. As the last of the Llandovery outcrops are traced towards the north, fossils become very rare indeed and, although towards the boundary line of the northern and central belts no specimens of D. vesiculosus or of D. acuminatus have been found, a few other graptolites which along the valley of the Tweed are associated with these zonal fossils, have been encountered. It is thus seen that within the remarkably short distance of 9 miles, as traced from the Dobb's Linn anticline to the Llandovery-Tarannon border, the whole of the Upper and nearly all of the Lower Birkhill shales have disappeared, the fossils becoming rare even in the shale members which,are found, and, most significant of all, the Tarannon grits or conglomerates everywhere follow upon whatever member of the Birkhill group forms the top of the section.

Such a stratigraphic relation might be interpreted in one of two ways. On the one hand it might be supposed that the Llandovery sea retreated to the southwest and that dry land conditions accompanied by subaerial denudation obtained in the areas laid bare. This would imply that more of the Birkhill shales had been deposited to the northwest of Moffatdale than are now seen in the sections and that the present exposures represent merely the parts which have not been touched by erosion. The Tarannon would then represent the river deposits spread out upon the eroded remnants of the Llandovery. That such is in all probability not the case is indicated by the statement made by Peach and Horne that in the Hartfell section (in the Frizzle Burn) "the black shales and mudstones of the Monograptus gregarius zone pass conformably upwards into the massive grits of Tarannon age without any representative of the Upper Birkhill Shales" (P. and H. 215, 133). The significant word is conformable. If the contact is conformable there was no erosion, and therefore it is not likely that two miles distant there was any considerable erosion. Thus another interpretation is called for. The facts, and they have