444 



HISTORICAL GEOLOGY 



Igneous Rocks. — The Silurian was a period of quiet as far as vol- 

 canism was concerned. In North America some igneous intrusions 

 of this age are known, but they are not extensive. 



Other Continents. — The Silurian epicontinental seas of Europe 

 were extensive and had much the same position as in the Ordovician. 

 Two distinct seas, one in the north and the other in the southern 

 part of the continent, were separated by a land ridge. The life of 

 the two seas was unlike in many particulars, that of the northern sea 

 being typical of the period in other continents, while that of the south- 

 ern had many peculiarities which indicate that it was partly inclosed. 



Life of the Silurian 



Aside from a notable increase in the number and variety of corals, 

 crinoids, brachiopods (spire bearers), and eurypterids, and a decrease 

 in the graptolites and straight cephalopods, the life of the Silurian did 

 not differ greatly in general aspect from that of the Ordovician. 

 When, however, one looks for identical species and genera, he finds 

 that the change was a marked one. 



Ccelenterata 



Corals forged ahead and became important in the Silurian. In- 

 stead of the few, usually simple forms of the Ordovician, a varied and 





ifc£X-Jti&flMHHHL BCD E 



FlG. 413. — Silurian corals: A, chain coral, II aly sites catenulatus; B and C, cup 

 orals, Strcptelasma {Enterolasma) calicula; D, Syringopora retiformis; E, Goniophyl- 

 lum pyramidale. 



abundant coral fauna, many of the corals compound, throve in the 

 dear seas of the time. Four well-marked types were abundant : (1) 

 chain corals (Halysites, Fig. 413 A), made up of vertical tubes joined 

 ther in such a way as to give them the appearance of a linked 

 chain. Since chain corals began in the Ordovician and became extinct 

 in t lit basal Devonian, their presence in a formation shows it to be 

 either Ordovician or Silurian. (2) Honeycomb corals (Favosites) were 

 composed oi six-sided parallel columns, like a honeycomb, which 



