Page:The Habitat of the Eurypterida.djvu/181

Rh upon Silurian, Ordovician or even older strata, including the metamorphic rocks of the Southern Highlands of Scotland. What has been taken as the Caledonian Old Red in the cases where it has been supposed that a passage exists is in reality a series of quite different age" (Goodchild, 80, 598, 599). As further evidence of the great break between the two systems Goodchild adds that the Lanarkian rocks shared in all of the tremendous disturbances to which the Siluric rocks were subjected and that "these disturbances had ceased, and had been followed by prolonged denudation, long before the oldest member of the Caledonian Old Red was laid down. Hence it results that the great unconformity, so often referred to, passes above what is left of the Lanarkian rocks. There is no clear evidence of any unconformity below them" (Goodchild, 80, 599).

Thus from the many sections described in the Scottish literature and especially from the authoritative statement of Goodchild, there seems to be good reason for believing that there was a great unconformity at the end of the Siluric, caused in part by profound tectonic disturbances, and that following upon these there was a long period of erosion before the earliest of the Caledonian deposits were laid down. These were of great thickness, amounting in some places to 20,000 feet. As to the origin of the series Goodchild says: "There appears to be evidence of a satisfactory nature that the whole of the vast formation was accumulated under continental conditions, partly in large inland lakes, partly as torrential deposits of various kinds, partly as old desert sands, and partly as the results of extensive volcanic action" (80, 596).

A brief review of the lithological characters and distribution of the Caledonian Old Red series will show most clearly that the rocks throughout are of continental origin. The lowest member, division 1, consisting of sandstones and conglomerates, is often wanting altogether, the overlying volcanics being the first of the series to be present. At the Falls of Clyde, near Lanark, Lanarkshire, these lower beds are, however, to be seen, and they are also found in a few other localities. Generally, the volcanics rest immediately and with a violent unconformity upon various pre-Devonic formations. It is these lavas which are seen in the Ochils and Sidlaw Hills, in the Pentlands and in the vicinity of Oban, at St. Abb's Head and also in the Cheviot Hills. In their greatest development in the Perth and Forfar Hills the volcanics may well reach several thousand feet in thickness, but they thin away toward the north and northeast and pass into