CHAPTER II 

 LOWER PALAEOZOIC FAUNAS (Pis. ix. AND x.) 



(A) GENERAL ACCOUNT 



THE strata of the Cambrian, Ordovician and 

 Silurian periods exhibit, in Britain, a certain 

 monotony of lithological quality, relieved locally, 

 and at various horizons, by development of volcanic 

 rocks. These interludes were most widely distributed, 

 and produced the greatest bulk of pyroclastic and other 

 effusive materials, in the Ordovician period ; but in 

 spite of the scarcely fossiliferous nature of the Arenig 

 series of Wales and the Borrowdale series of the Lake 

 District, a practically continuous faunal record can be 

 discerned, in one district or another, throughout the 

 Lower Palaeozoic era. The dominant type of sediment 

 is best expressed by the term " Greywacke " detrital 

 matter with an average composition of sandy slate 

 containing small quantities of drifted volcanic ash. 

 Actually all ordinary grades from conglomerate to 

 pure clay-slate occur, but a certain argillaceous quality 

 pervades the whole series, except in the north-west 

 Highlands of Scotland. With the same exception, 

 the few, relatively thin and impersistent, limestones of 

 the series are usually earthy, even the coral-reefs being 

 impregnated with grey shale. These limestones are 

 either coralline or shelly ; in no cases do they appear 

 to represent deposits of the open sea. In many 

 districts, notably the Welsh border and Anglesea, the 



