Ch. XXVII.] 



WENLOCK FOEMATION". 



435 



Fig. 579. 



Wenloch formation. — We next come to the Wenlock formation, which 

 has been divided (see Table, p. 430) into the Wenlock hmestone and the 

 Wenlock shale. 



1. The Wenlock limestone, formerly well known to collectors by the 

 name of the Dudley limestone, forms a continuous riclge in Shropshire, 

 ranging for about 20 miles from S. W. to N. K, about a mile distant from 

 the nearly parallel escarpment of the Aymestry limestone. This ridgy 

 prominence is due to the solidity of the rock, and to the softness of the 

 shales above and below it. ISTear Wenlock it consists of thick masses of 

 gray subcrystalline limestone, replete with corals and encrinites. It is 

 essentially of a concretionary nature, and the concretions, termed " ball- 

 stones" in Shropshire, are often enormous, even 

 80 feet in diameter. They are of pure carbo- 

 nate of lime, the surrounding rock being more 

 or less argillaceous.^ Sometimes in the Mal- 

 vern Hills this limestone, according to Professor 

 Phillij^s, is oolitic. 



Among the corals in which this formation is 

 so rich, the " chain-coral," Halysites catenula- 

 tus^ or Caienipora escharoides (fig. 579), may 

 be pointed out as one very easily recognized, 

 and widely spread in Europe, ranging through 

 all parts of the Silurian group, from the 

 Aymestry limestone to near the bottom of the 



series. Another coral, the Favosites Goth- maysitescatenuiatm,Unn.^^. 

 landica (fio-. 580), is also met with in profusion J>j'^- gatenipo,-a escharoides, 



^ ^ , ■' , , ^ (jold. Upper and Lower Silurian. 



in large hemispherical masses, which break up 



into prismatic fragments, hke that here figured (fig. 580). Another 

 common form in the Wenlock hmestone is the Omphyma (fig. 581), 

 which, like many of its companions, reminds us of some modern cup- 

 corals, but all the Silurian genera belong to the paleozoic type before men- 



Fig. 5S0. 



Fig. 581. 



Favosites Gothlandica, Lam, Dudley. 

 a. Portion of a large mass; less than the 



natural size. 

 &. Magnified portion to shoAv the pores 



and the partitions in the tuhes. 



Omphyma turMiiatum, Linn. sp. 



(Cyathophyllum, Goldf.) 

 Wenlock Limestone, Shropshire. 



* Murchison's Siluria, p. 115. 



