Page:The Habitat of the Eurypterida.djvu/132

been gleaned from a detailed study of many more sections than can be mentioned here, lead to the conclusion that there was no erosion between the deposition of the last of the Llandovery (Birkhill) beds, whether they were Lower or Upper Birkhill, and the lowest Tarannon beds. The structural relation is, therefore, one of replacing overlap, the Tarannon beds pushing to the southeast just as rapidly as the graptolite-bearing Llandovery muds retreated in the same direction. Lines of sections at right angles to the strike of the Llandovery and Tarannon rocks taken in various places from coast to coast, indicate that in the northern part of the central belt the Tarannon is always of a massive unfossiliferous character, grading southeastwards into graptolite-bearing shales and mudstones. There is no doubt that the coarse conglomerates and grits were river-borne. In one of the typical localities in the Moffat district Peach and Horne, in describing the conglomerate say, "the rock possesses a greywacke matrix, in which are embedded rounded pebbles of quartz, red chert with radiolaria, Arenig volcanic rocks, with boulders of granite and quartzite from eight to ten inches in diameter. Some small pieces of micaschist have also been observed. The fragments of quartzite and micaschist resemble rocks of those types in the Eastern Highlands; there can be little doubt that they were derived from that region" (P. and H. 215, 210). These authors also note in regard to the greywackes and grits that, "both volcanic and plutonic rocks have contributed to their formation. The fragments are angular or subangular. Well-rounded grains are rare. There is, further, a very great variability in the sizes of the constituent grains; indeed, the material does not appear to have been well sorted by aqueous action" (215, 211). It seems surprising that materials which had been transported by rivers for so great a distance, it being about one hundred miles from the Eastern Highlands to the Moffat district, should not be better sorted. However, it is clear that the material must have been brought down by rivers. That it was deposited as a subaerial delta or series of deltas which spread out into the sea to the southwest is suggested by the character of the materials, for in the extreme north of the Tarannon area there are only the coarse conglomerates without fossils, but these deposits merge ever so slowly into finer ones southward, the first change in the conditions of sedimentation being indicated by the intercalation of thin, leaf-like beds of shales bearing graptolites. Indeed, this domination of terrestrial over marine sediments is seen even towards the close of the Llandovery in many