SILURIAN SYSTK.M 191 



cases abruptly against another kind of limestone. The Swedish 

 geologists call these beds reefs, and apply the term not only to 

 such beds as are largely made up of corals, but also to those of 

 which the principal constituents are Stromatoporoids or Bryozoa. 

 Thus, writing of the Ascoceras limestone, Munthe says, " It may be 

 considered as a reef-limestone formed chiefly of Stromatopora and 

 of large fragments of Crinoids with marly sediment to fill the 

 interspaces," and it should be noted that the Cephalopod Ascoceras 

 is common in this limestone. 



Good descriptions and photographs of these beds are given in the 

 livret-guicles provided for the Geological Congress at Stockholm in 

 1910 and from them it is plain that by the term "reef-limestone" 

 the writers merely mean limestones composed of organic remains 

 which have been accumulated in such a manner as to be unstratified. 

 One such bed is shown to be overlain by stratified Sphaerocodium 

 limestone, another by crinoid limestone, and massive colonies of 

 Stromatoporoids sometimes replace crinoid limestone. It is evident 

 that all the beds were formed in shallo\v water where the sea-floor 

 was swept by strong currents, to the action of which the frequent 

 abrupt termination of some beds and their replacement by others 

 may be ascribed, but it is very doubtful whether any of them ever 

 formed a reef in the proper sense of that term. 



REFERENCES 



1 C. Lapworth, "On the Geological Distribution of the Rhabdophora, " 

 Ann. and Mag. Nat. Hist. ser. 5, vol. v. and vi. (1880). 



2 Miss E. M. Wood, Quart. Journ. Geol. Soc. vol. Ivi. p. 415 (1900). 



3 G. Maw in Geol. Mag. for 1881, p. 100 ; and C. Lapworth in Proc. 

 Geol. Assoc. vol. xiii. p. 324. 



4 The Misses Elles and Slater, Quart. Journ. Geol. Soc. vol. Ixii. p. 195 

 (1906). 



5 H. Lapworth, Quart. Journ. Geol. Soc. vol. Ivi. p. 67. 



" The Misses Elles and Wood, Quart. Journ. Geol. Soc. vol. lii. p. 273 

 (1897). 



7 Miss G. L. Elles, Quart. Journ. Geol. Soc. vol. Ivi. p. 370. 



8 Miss E. M. Wood, Quart. Journ. Geol. Soc. vol. Ivi. p. 415. 



9 P. Lake, Quart. Journ. Geol. Soc. vol. li. p. 9. 



10 T. M'K. Hughes, Quart. Journ. Geol. Soc. vol. xxxv. p. 694. 



11 H. Hicks, Quart. Journ. Geol. Soc. vol. xxxvii p. 482. 



12 Miss G. Elles, Quart. Journ. Geol. Soc. vol. Ivi. p. 169. 



13 A. Strahan, Quart. Journ. Geol. Soc. vol. xxxv. p. 268. 



14 Messrs. Marr and Nicholson, Quart. Journ. Geol. Soc. vol. xliv. p. 658. 

 5 Miss G. L. Elles, Quart. Journ. Geol. Soc. vol. liv. p. 463 (1898). 



18 Misses Watney and Welch, Quart. Journ. Geol. Soc. vol. Ixvii. p. 215 

 (1910). 



17 C. Lapworth, Quart. Journ. Geol. Soc. vol. xxxiv. p. 240 and vol. 

 xxxviii. p. 587. 



