150 STEATIGRAPHICAL GEOLOGY 



the metamorphic complex of the Central Highlands are Ordovician 

 rocks is still held tenaciously by certain authorities the progress of 

 geological research has made this theory more and more improbable. 

 There is therefore good reason to suppose that no such rocks are 

 there, and if this is so the Atlantic land probably extended over the 

 greater part of Scotland, as indeed I haA r e ventured to indicate 

 elsewhere (Building of the British Isles'). 



Lastly there must also have been a land-area of considerable 

 size in a more southern part of the Atlantic to the west of France 

 and Spain, for it is only from that direction that the materials 

 constituting the Ordovician deposits of those countries can have 

 been derived. The felspathic sandstones and conglomerates of 

 Brittany show that part of this land extended very near to France, 

 and also that it consisted very largely of granitic and Archaean 

 rocks. 



The greater part of the Ordovician sediments are of the normal 

 detrital character conglomerates, sandstones, flagstones, grits, 

 mudstones, and shales, the materials of which must have been 

 derived from the land tracts which existed in the positions above 

 indicated. 



Limestones were also formed wherever the water was clear 

 enough to permit shell-bearing creatures to live and flourish in 

 great numbers, so that their shells and tests accumulated to form 

 layers on the sea-floor. The chief components of such limestones 

 are Mollusca, Brachiopoda, Crustacea, Bryozoa, and Cystidean 

 Echinoderms. A few genera of corals also contributed to their 

 formation, but do not seem to have been very abundant. 



Besides these sediments some of the Arenig shales include layers 

 and nodules of chert which contain remains of Eadiolaria, and 

 concerning these some erroneous ideas have arisen as to the 

 conditions indicated by the presence of Eadiolaria. This matter 

 has been discussed in the third edition of my Building of the 

 British Isles (1911), but it may be well to mention here that 

 those, however, who claim that the radiolarian cherts of this 

 region are "oceanic deposits" jump to a very important conclusion 

 from a very small amount of evidence. Only two reasons have 

 been given for such an inference : (1) the existence of the Radiolaria, 

 which at the present day form deposits on oceanic floors, (2) the 

 small thickness of the Arenig-Llandilo Beds ; but neither of these 

 ca*A be taken as evidence of oceanic conditions. As a matter of 

 fact Eadiolaria occur in waters of all depths, and have been found 

 in deposits of many ages, notably in the Oxfordian and in the 

 Lower Eocene of Northern France, in each case combined with 

 sponge spicules to form a peculiar siliceous rock termed gaize. 



