The Llandovery-Tarannon rocks of Southern Scotland show the two types which prevailed in the Moffat and Girvan areas in later Ordovician times. The Llandovery beds of Moffat are known as the Birkhill shales, and are very thin. The representatives of the Tarannon shales, however, the Gala beds, consist mainly of grits, and attain a great thickness. In the Girvan area, the Llandovery beds are of the shelly type. Here as at Moffat and in the Lake District there is perfect conformity between the beds of Ordovician and those of Silurian age, and accordingly it is instructive to note the completeness of the palæontological break, especially in the Moffat district. The higher Silurian beds of Southern Scotland present a general resemblance to those of North Wales and the Lake District[80]. [Footnote 80: For descriptions of the Silurian beds of the typical region see Lapworth and Watts, _Proc. Geol. Assoc._, vol. XIII. p. 297, those of Wales are described by Lake and Groom, _Quart. Journ. Geol. Soc._, vol. XLIX. p. 426, and Lake, _ibid._ vol. LI. p. 9. A description of those of Lakeland will be found in the Memoir of the Geological Survey "The Geology of the Country around Kendal, etc." while the Scotch Rocks are described in Lapworth's papers on Moffat and Girvan.] On the European continent we find indications of conditions similar to those which prevailed during the Ordovician period; the strata become much thinner and more calcareous in Scandinavia, and still thinner in the Baltic provinces of Russia, where they consist very largely of calcareous matter. In central Europe the greater abundance of calcareous matter, compared with that which is found in the Ordovician strata of that region, points to a change in physical conditions which became still more marked after Silurian times. In North America, the succession is very similar to that of Britain, the calcareous development of the Silurian rocks being found around Niagara, but towards the close of Silurian times the shallow-water phase became marked in places by the deposition of chemical precipitates which indicate the separation of a portion of the late Silurian ocean from the main mass during the period of formation of these abnormal deposits.