226 



Murchisoris Silurian System. 



called ballstones,* sometimes of immense size, surrounded 

 by beds of shale and impure limestone. 



Beneath these beds, a shale containing concretions of 

 very impure limestone occurs. These beds, which are called 

 Wenlock shale, constitute the base of the upper Silurian 

 rocks. These beds are succeeded occasionally by courses of 

 very impure lenticular limestone, the concretions of which 

 contain concentric figures, made up of dark coloured crystal- 

 line carbonate of lime in an argillaceous paste. 



The Wenlock portion of the upper Silurian rocks is esti- 

 mated by Mr. Murchison at about 1000 feet in thickness; 

 that is, 300 feet for the limestone, and 700 for the beds of 

 shale. 



The minerals found in it, are calcspar, sulphate of ba- 

 rytes, lead and iron, peroxide of manganese, sulphurets of 

 copper and bitumen, but not in such abundance as to render 

 any of them of much value. 



The organic remains on which the peculiarity of this rock 

 depends, consist of 



Corals. 



Heliopor a piriformis, (De Blain.) 

 Catenipora escharoides, (Lamarck.) 

 Stromatopora concentrica, (Goldf.) 

 Favosites Gothlandica, (Lamarck.) 

 Cyathophyllum turbinatum, (Goldf.) 

 Simaria clothrata, (Steininger.) 



Conchifers. 

 Euomphalus discors, 



rugosus. 



funatus, 



Productus Euglyphus, 

 depressus, 



* These are used as a flux for iron ore, and preferred, Mr. M. remarks?, 

 to the impure limestone. 



