150 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 25, 



After these remarks, we are at once prepared to compare the rocks 

 developed in the great transverse sections of the "Welsh and Cumbrian 

 mountains. The lower Cambrian groups (Bangor and Festiniog, 

 Nos. 1 and 2 tabular view) are amply represented by the green slates 

 and porphyries of Cumberland. The upper Cambrian groups (Bala and 

 Caradoc, Nos. 3 and 4) are (however imperfectly in thickness) clearly 



Diagram illustrating the Comparative Development of the Silurian 



and Cambrian Rocks in Wales and Cumbria, respectively, 



Cambria. Cumbria. 



Upper PALiEozoic 



Middle PALiEozoic, 



Ludlow 



Wenlock 



Caradoc . . 



Upper Bala ... 

 Bala Limestone 



/Carboniferous Lime- 



l stone. 



Old Red Sandstone. 



Kirkby flags. 

 Coarse slates. 

 Ireleth slates. 



Coniston grits. 



, . Coniston flagstone. 

 . Coniston limestone. 



Lower Bala 



Slates and porphyry* 



Arenig slates and por- &: 

 phyry.... 





Tremadoc slates . . 



Lingula flags 



Harlech grits 



Llanberris slates . . , 







Metamorphic 

 Granite 





. SMddaw slates. 



Metamorphic. 

 Granite. 



represented by the Coniston limestone, flagstone, and the hard coarse 

 grits of the Westmoreland sections. 



The equivalents, in the North of England, of the Wenlock and 

 Ludlow groups (Nos. 5 and 6) have already been noticed. Using the 

 words "Silurian System " in any definite sense, these are the groups 

 which truly and exclusively belong to it as a system ; for the Caradoc 



the characteristic Wenlock species ; while the so-called Caradoc sandstone of May 

 Hill contains the Wenlock fossils in abundance, and none of the characteristic 

 Cambrian types. But is there a single section in which these two distinct groups 

 of fossils appear together in one stage ? If no such section can be found, why may 

 we not suppose that the Caradoc sandstone of May Hill is a group superior to the 

 Caradoc sandstone of Horderley ? Should we ever be able to answer this question 

 in the afBrmative, the ambiguity alluded to in the text would be at an end. The 

 statement here given is drawn from the fossil evidence supplied by the Cambridge 

 Museum, 



