518 



GEOLOGY 



parts of Europe, and they exist concealed by younger formations 

 over considerable areas where they are not seen. Fig. 384 repre- 

 sents the general geographic relations of land and water in Europe 

 during this period. The submerged area represents in a general way 

 the area where the Ordovician formations are present. 



Fig. 383. Shows modes of occurrence of the phosphates (the shaded sur- 

 face parts of the limestone) in central Tennessee. (Hayes and Ulrich, 

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



The formations of the European Ordovician are largely frag- 

 mental, being made up of shales, sandstones, graywackes, etc., with 

 which there is associated relatively little limestone. In this respect 

 the Ordovician of Europe is in contrast with that of North America. 



The system is represented in the British Isles by great thick- 

 nesses of strata (something like 24,000 feet maximum). 1 Locally 

 (Wales), nearly half the system is composed of igneous rock, con- 

 sisting of sheets of lava and beds of fragmental igneous rocks of 

 various sorts. In the north of England, the successive beds of 

 igneous rock, partly lava-flows and partly tuffs, not interstratified 

 with aqueous sediments except near the base and summit, su<rirest 

 that the eruptions took place, in part at least, on land. In Wales, 



1 This measurement is doubtless subject to the strictures set forth on p K.l. 



