84 SILURIAN. 



SIL URIA N. 



UPPER SILURIAN. (Murchison and Geol. Survey.) 



The term Silurian System, with which the name of Sir 

 Roderick I. Murchison will ever be connected, was given in 

 1835 from the country of the ancient Britons known as 

 Silures ; a tract extending over the south-eastern portion 

 of Wales and adjoining parts of England, Murchison 

 commenced his labours in this field in 1831. And later on 

 he tells us that, " When Ostorius, the Roman general, con- 

 quered Caractacus, he boasted that he had blotted out the 

 very name of Silures from the face of the earth. A British 

 geologist had, therefore, some pride in restoring to currency 

 the word Silurian, as connected with great glory in the annals 

 of his country."^ The term was not altogether appropriate, as 

 it indicates but a small portion of the area over which Silurian 

 rocks are developed, and it is, for the reason stated previously, 

 here confined to the rocks between the Bala Series and the 

 Lower Old Red Sandstone. The beds rest unconformably 

 upon the Cambrian rocks.^ 



The total thickness of the system may be as much as 

 14,000 feet in the north-west of England, while it varies 

 from 3000 to 6000 feet in Wales, for the beds them- 

 selves are subject to much local change. They comprise 

 slates, shales, and grits, with important beds of Coral- and 

 Encrinital-limestone. It has been observed that all the 

 Silurian limestones are local phenomena ; where the Wool- 

 hope limestone is well developed, as at Presteign, there the 

 Wenlock limestone is very feebly represented. At Wenlock 

 the limestone forms a grand terrace, but the Aymestry lime- 

 stone has almost vanished. At Leintwardine the Aymestry 

 limestone is a grand rock, and the Wenlock limestone is but 

 poorly developed. 



The organic remains of the Silurian rocks indicate their 

 marine origin, and show that the beds were deposited in a 

 continuously but slowly subsiding area, and in water never 

 of very great depth. There are no evidences of volcanic 

 action during this period in England and Wales. 



^ P. Geol. Soc. iii. 640. See also Silurian System, 1839. Q. J. viii. 173. 

 ^ See Horizontal Sections, Geol. Survey, Sheets 2il and 34. 



