UPPER PALAEOZOIC FAUNAS 155 



of its included fossils during dolomitization ; but the 

 enclosed and " unhealthy " quality of the sea in which 

 it was formed doubtless limited the numbers, as it 

 dwarfed the individuals, of the indigenous fauna. 

 Further, the bulk of British Devonian rocks (restricted 

 in outcrop to Devonshire and Cornwall) has been 

 caught in the toils of post-Carboniferous earth-move- 

 ment and igneous activity, and is in consequence strati- 

 graphically complex and lithologically obscure. The 

 only Upper Palaeozoic series that is adequately fossili- 

 ferous (for Invertebrates) in Britain is the Lower Car- 

 boniferous. Many of the shales and limestones of 

 that age compensate, by the abundance and good pre- 

 servation of their faunas, for the unsatisfactory nature 

 of palaeontological evidence at the other horizons. 



The upheaval that brought the Silurian period to a 

 close in the Downtonian stage resulted in development 

 of an extensive area of land covering (with the excep- 

 tion of local and probably temporary lakes) all of the 

 British area north of the " Mendip axis." In the 

 hollows of this land-surface, the boulder-beds and 

 desert-marls of the Old Red Sandstone were mingled 

 with the products of contemporary volcanic eruptions. 

 In the littoral belt of sea bordering the continent, 

 detrital deposits accumulated in a lagoon whose 

 southern boundary was, during part of the period, a 

 fringing coral - reef. The Carboniferous period com- 

 menced with the breakdown of the "Mendip" shore- 

 line, and consequent northward encroachment of the 

 Devonian sea. Before this tendency was checked, 

 most of England and Ireland (save for local islands 

 of which " Wales " was the chief) became submerged, 

 and the Carboniferous Limestone was accumulated. 

 Towards the Cheviots the purity of the sea-water de- 

 clined, under influence of the surviving " highlands " 



