340 GEOLOGY. 



markedly different in closely associated localities, and even the larger 

 divisions established for the northern province do not hold for the 

 southern. 



The Ordovician system was first differentiated in Great Britain, 

 where it was described under the name of Lower Silurian, 1 a name 

 which is still frequently applied to it. The name here used for the 

 system, which comes from the region occupied by the Ordovices, is of 

 relatively recent adoption. 2 



The system is represented in the British Isles by great thicknesses 

 of strata (something like 24,000 feet maximum). 3 Locally (Wales), 

 nearly half the system is composed of igneous rock, consisting 

 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, suggest that the eruptions 

 took place, in part at least, on land. In Wales, on the other hand, 

 the igneous rocks are interstratified with sedimentaries, and are there- 

 fore thought to have been ejected beneath water. 4 This is one of 

 the most extensive, as well as one of the most ancient, volcanic tracts 

 of Europe. Igneous rocks of Ordovician age also occur in the north 

 of Ireland. 



From north England and Wales the system thins in all directions, 

 though it has a thickness of something like three miles in the southern 

 highlands of Scotland. The figures given are to be looked upon as 

 maxima, not as averages. In Scandinavia and Russia the system 

 has but a fraction of the thickness which it possesses in Britain, and 

 the strata are essentially horizontal. About the Gulf of Finland, some 

 of the Ordovician beds of clay and sand are still unconsolidated. The 

 sands and clays of the system here might be taken, so far as lithological 

 characteristics go, for Tertiary. 



In the southern province of Europe, as in most of the northern, 

 the Ordovician system does not attain great thickness, and limestone 

 is there more abundant than in the north. The strata are exposed 

 about various mountains where local disturbances have upturned 



1 Murchison, Phil. Mag. and Brit. Assoc, 1835. 

 2 Lapworth, Geol. Mag., 1879, p. 13. 



3 This measurement is doubtless subject to the qualifications set forth on p. 259. 



4 Geikie, op. cit., pp. 946 and 949. 



