572 GEOLOGY 



imconformable on its base. This is especially true in the north- 

 western part of the continent. 



During the progress of the period, the European continent was 

 progressively submerged, for the Middle and Upper Devonian forma- 

 tions are more wide-spread than the Lower (Fig. 413). In this 

 respect, the phenomena of Europe are in partial correspondence 

 with those of America, though the overlap of the Middle and Upper 

 Devonian of Europe is greater than in America. 



The British Isles. In the British Isles the Devonian system 

 has two phases. The first is found in southwestern England in the 

 area which gave the system its name (Devonshire). The system 

 here is thick and of marine origin. Igneous rocks are often asso- 

 ciated with the sedimentary. Valuable ore-bearing veins occur, 

 as in Devon and Cornwall. 



The second phase of the Devonian is the Old Red Sandstone, 

 widely distributed in Great Britain and Ireland and found at some 

 points on the continent. Concerning the history of this Sand- 

 stone there has been much difference of opinion, but it is commonly 

 held to have been deposited in a series of inclosed or nearly inclosed 

 basins containing lakes or inland seas, the waters of which were 

 fresh or brackish. It is further believed that crustal warpings 

 gave the sea occasional access to these basins. In general the 

 strata are poor in fossils. Some of those present are fresh-water 

 species, and others are land species; but marine species occur at 

 some horizons. It is not improbable that some parts of this singu- 

 lar sandstone are of subaerial, rather than subaqueous origin. The 

 Old Red Sandstone has some features like those of the Catskill 

 formation of America. 



The Old Red Sandstone of the British Isles has at its maximum 

 a thickness of more than 20,000 feet, 1 but this includes much 

 igneous rock. There is an unconformity in the series, and the upper 

 division contains conglomerates of such a character as to have 

 raised a question concerning the existence of glaciers in Great 

 Britain in Devonian times. 



West central Europe. The Devonian of Germany is remarkable 

 for the proportion of igneous rock interbedded with the sedimen- 



1 See notes concerning such thicknesses, p. 461. 



