Page:The Habitat of the Eurypterida.djvu/128

 it. While the faunas and the lithological deposits in England and Wales indicate, with few exceptions, the prevalence throughout the Ordovicic of open marine conditions, in southern Scotland, on the other hand, the record is one of oscillations, showing now the prevalence of terrigenous deposits, again that of sea-derived or thalassigenous deposits.

A rapid survey of the succession of events during Ordovicic time shows that there was a gradual retreat of the sea towards the south and southeast during the middle and upper Ordovicic and the lower Siluric, followed by a widespread advance during Wenlock time. A few of the typical sections will readily bring out these facts (see also the general description of the region on p. 151).

The Ordovicic and Siluric rocks of the Southern Uplands of Scotland are exposed in a series of belts trending northeast-southwest. The southernmost is a rather narrow, discontinuous strip composed of Wenlock and Ludlow flaggy grits and mudstones, bordering the northern coast of Solway Firth and extending northeast into the Cheviot Hills. The second belt, from 20 to 25 miles wide extends from St. Abbs Head on the east coast, through the Lammermuir Hills across the greater part of Selkirk, Peebles, Dumfries, Kirkcudbright and Wigtown (see map). This band consists of the Lower Siluric Llandovery and Tarannon beds. The third belt, narrow in the east where it does not quite reach the coast, but constituting the northern slopes of the Lammermuir Hills, broadens westward until it becomes 15 or more miles wide. It consists of Llandeilo and Caradoc limestones with a large amount of radiolarian chert of Arenig (Lower Ordovicic) age. The northwestern termination of this belt is the Girvan area with its great development of Arenig volcanic rocks. From 5 to 10 miles north of the third belt are two important regions one in the Pentland Hills, Edinburghshire, the other in Lanarkshire, where the Wenlock, Ludlow and Downton beds are exposed as inliers in the Old Red sandstone. The relation of these isolated Siluric outcrops to those of the southern tableland will be made clear by a consideration of the tectonic arrangement.

Towards the close of the Lanarkian a pronounced uplift took place accompanied by a tremendous amount of lateral compression giving a great series of folds whose axes run northeast-southwest, parallel to the major axis of the tableland. Denudation set in before the beginning of Old Red deposition so that the Old Red rests unconformally upon Siluric or Ordovicic beds. Moreover, formations which