FORMATIONS AND PHYSICAL HISTORY 41 1 



nrvntly for the reason that anticlines furnish an inverted basin 

 of holding these substances against the pressure of the 



Rl-'itf. 361. Section showing the relations of the Devonian and other Paleozoic 

 cms in the \ it inity of Loudon, Tenn. -C = Cambrian; = Ordovician; S = Si- 

 luii.in; /;= Devonian; </;< = age unknown. Length of section, about 7 miles. 

 (Keith, U. S. Geol. Surv.) 



heavier subterranean water which tends to force them to the sur- 

 face. In all cases it appears that there must be impervious beds 

 above to prevent the escape upward of the oil and gas. 



The Devonian of central Tennessee is the horizon of black 

 phosphates, which are of importance commercially. 1 



Foreign Devonian 



Europe. At the close of the Silurian there seem to have been 

 more considerable geographic changes in Europe than in America, 

 for the Devonian system there is more commonly unconformable 

 on its base. During the progress of the period, Europe was pro- 

 gressively submerged, for the Middle and Upper Devonian forma- 

 tions are more widespread than the Lower (Fig. 362). 



In the British Isles the Devonian system has two phases. The 

 first is found in the area which gave the system its name (Devon- 

 shire). The system here is thick and of marine origin. Igneous rocks 

 are associated with the sedimentary, and the system has valuable 

 ore-bearing veins, 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 believed 

 to have been deposited in a series of inland lakes or seas, the waters 

 of which were fresh or brackish. Since species of marine fossils 

 occur at some horizons, the sea had access to the basins at times. 

 It is not improbable that some parts of this singular sandstone are 

 of subaerial, rather than subaqueous, origin. The Old Red Sand- 

 stone has some features like those of the Catskill formation of 

 America. In the British Isles, the Old Red Sandstone has great 

 thickness and includes much igneous rock. 



1 Columbia (Tenn.) folio, U. S. Geol. Surv. 



