CHAPTER XVI. 



SILURIAN SYSTEM. 



W*E have at length reached those older deposits, which not having been separated 

 into formations by previous writers, I am compelled to describe under new terms. 



Acting upon the principle which guided William Smith in subdividing the oolitic 

 system of our island, I have named these rocks from places in England and Wales, 

 where their succession and age are best proved by order of superposition and im- 

 bedded organic remains, and have termed them in descending order, the " Ludlow", 

 " Wenlock",." Caradoc", and "Llandeilo" formations. The same principle has led 

 me to use the general term of Silurian System for the group, to mark thereby the ter- 

 ritory in which the best types and the clearest relations are exhibited 1 . 



Like every other mass of strata entitled to the name of System, the Silurian, though 

 clearly recognisable as a whole over extensive tracts, cannot always be subdivided into 

 those formations which are displayed in, the regions where I shall first describe it, and 

 where its types are fully developed. Thus, for example, where the subordinate limestones 

 thin out and disappear, the Ludlow deposit can seldom be clearly separated from that of 

 Wenlock. In such cases both these formations are included in the term of £C Upper 

 Silurian Rocks", and under similar circumstances the Caradoc sandstones and Llan- 

 deilo flags, in that of " Lower Silurian Rocks". 



Simple as this classification may now appear, those versed in practical geology can 

 well understand what must have been the amount of examination employed in its perfect 

 establishment. To comprehend the extent of the break in the history of the older strata 

 which has been filled up by the study and classification of these rocks, the student has only 

 to refer to the tabular view I have prepared, and compare it with other tables framed 

 upon an antecedent state of geological knowledge. He will then perceive, that what 

 is here presented to him, as a well-ordered succession of great thickness, (each subdi- 



1 The country occupied by this system embraces large tracts of England and Wales, the principal part of 

 which was included in the ancient British kingdom of the Silures : hence the name I have selected. See intro- 

 ductory chapter ; Appendix ; and Table of Superposition. In the latter, the names of the overlying formations 

 are printed in ordinary characters, whilst those of the older rocks, to which new names have been assigned, 

 are in italics. 



