122 THE HABITAT OF THE EURYPTEEIDA 



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 preva- 

 lence of terrigenous deposits, again that of sea-derived or thalassige- 

 nous 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 Scot- 

 land 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 north- 

 ern 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 Llan- 

 dovery and Tarannon beds. The third belt, narrow in the east 

 where it does not quite reach the coast, but constituting the north- 

 ern slopes of the Lammermuir Hills, broadens westward until it be- 

 comes 15 or more miles wide. It consists of Llandeilo and Caradoc 

 limestones with a large amount of radio larian 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 uncon- 

 formally upon Siluric or Ordovicic beds. Moreover, formations which 



