CHAPTEE IV. 



SILURIAN PERIOD. 



1. Stratigraphical Evidence. 



COMMENCING- with what is usually regarded as the 

 typical Silurian district, it may be stated that the 

 Silurian rocks form a continuous escarpment through 

 Shropshire and the east of Wales, and that they rise to the 

 surface at intervals in the counties of Gloucester, Mon- 

 mouth, Hereford, and Stafford. 



The lowest beds in this district are sandstones, shales, 

 and conglomerates ; this (Llandovery) division is thickest 

 in the western part of the area, and is there divisible into 

 two groups, with a combined thickness of nearly 2,000 

 feet, but the upper stage frequently overlaps the lower, 

 and it is the only one found in the more eastern outcrops, 

 where they include beds of conglomerate ; these basal con- 

 glomerates vary much in thickness, and sometimes overlap 

 the Ordovician strata, as in the Longmynd and Malvern 

 areas, where they rest upon the Cambrian rocks. 



The Llandovery sandstones are succeeded by a great 

 series of dark grey shales, in which limestones are developed 

 at intervals ; some of these limestones attain a thickness 

 of 100 feet, but they are all lenticular deposits, and it is 

 often difficult to identify the beds seen in one detached 

 area with those which occur in the others. Only three of 

 these limestone bands have received distinctive names, 



