Page:The Habitat of the Eurypterida.djvu/130

 in the northern belt of Arenig rocks indicates that the shore was not far distant. There is evidence that during Llandeilo time conditions were less stable in the northern area, for the black graptolite-bearing Glenkiln shales (Upper Llandeilo) often merge laterally into greywackes and grits, while sometimes, as for instance in sections at the headwaters of the Girvan River, the Glenkiln fossils occur in "minute dark seams in sandy shales, embedded in massive greywackes and grits" (Peach and Horne 215).

The section which is most complete, showing no disconformities and indicating, therefore, continuous deposition, is that at Moffatdale about 10 miles to the northeast from Moffat, where in the famous

Dobb's Linn anticline studied by Lapworth, the succesions given in the first column, figure 10, is shown. It will be seen that the Glenkiln and Hartfell groups (Llandeilan and Caradocian, respectively), are complete and that the latter is followed by the Birkhill shales (Llandovery) which end with the Rastrites maximus zone (b3), which in turn is conformably followed by the green and grey shales of the Lower Tarannon. Crossing the strike to the northwest for about five miles, the Hartfell section is met with. It is the type locality for the shales of that name. The succeeding Birkhill shales are found to go no higher than the Monograptus gregarius zone (a3), which is conformably followed by the Tarannon grits. The