SILURIAN PERIOD. 515 



CHAPTER XLVIII. 



SILURIAN PERIOD. 



ROCKS OF THE PERIOD. 



FOLLOWING the Cambrian comes the great Silurian series of 

 rocks, first clearly established and definitely worked out by 

 Sir Roderick Murchison, the founder of the Silurian system. 

 The exact limit between the Cambrian and Silurian formations 

 is one which is not clearly defined, since there does not appear 

 to be any general physical break between the two groups. 

 The line of demarcation between them is, in the present state 

 of our knowledge, an arbitrary line, and is derived chiefly from 

 the characters of the Trilobites. There are rocks, however, 

 such as the Tremadoc Slates, the Skiddaw Slates, and the Cal- 

 ciferous and Quebec groups, in which there is an intermixture 

 of Cambrian with true Lower Silurian types. These rocks, 

 therefore, might be regarded as Upper Cambrian or as Lower 

 Silurian, or as passage-beds between the two. It is to be 

 remembered, also, that the Tremadoc Slates and Lingula Flags 

 are regarded by Sir Roderick Murchison as being the base- 

 ment-beds of the Lower Silurian. 



The name " Silurian " was proposed by Sir R. Murchison 

 for a great series of strata lying below the Old Red Sandstone, 

 and occupying those parts of Wales and England which were 

 at one time occupied by the " Silures," a tribe of ancient 

 Britons. The Silurian rocks are largely developed in Wales, 

 the north of England, Scotland, and Ireland, in various parts 

 of Europe, especially Bohemia, Saxony, Russia, and Sweden, 

 and in the North American Continent. The entire series is 

 divisible into the two sections of the Lower and Upper Silurian 

 rocks, each in turn split up into smaller subdivisions, the names 

 of which have usually been taken from localities where they are 

 unusually well developed, or where they were first studied. 



In Britain the Silurian Rocks are divided into the following 

 groups from below upwards : 



a. Lower Llandeilo group, -j 



b. Upper Llandeilo group, I e ilnr ; an 



c. Bala, Caradoc, or Coniston group, [ ^ 



d. Lower Llandovery group, ) 



e. Upper Llandovery group, \ 



f. Wenlock group, > Upper Silurian. 



g. Ludlow group, ) 



