xlii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



formities of its members, extending even to the Tarannon Shale, 

 that they were formed during an epoch of oscillation of level, and 

 therefore that its component parts only represent fragments of a 

 great intermediate epoch that elapsed between the close of the de- 

 position of the Caradoc Sandstone and the beginning of that of the 

 Wenlock Shale. 



"We have now seen that in the whole Silurian series there are six 

 very distinct sets of strata, and five stratigraphical breaks between 

 them, as follows : — 



Lingula-flags. 



Break very nearly complete both in genera and species, and 

 probable unconformity. " 



Tremadoc Slate. 



Break very nearly complete both in genera and species, and 

 probable unconformity. 

 Llandeilo and Caradoc beds. 



Large break, especially in species, and probable unconformity. 

 Lower Llandovery beds. 



Break and decided unconformity. 

 Upper Llandovery beds. 



Break and strong unconformity. 

 Wenlock Shale, &c. 



Each of these breaks, in my opinion, necessarily implies a lost 

 epoch, stratigraphically quite unrepresented in our area, and the life 

 of which is only feebly represented in some cases by the fossils 

 common to the underlying and overlying formations. But to this 

 important subject I shall return when I have summed up the evi- 

 dence respecting the breaks in succession in the higher members of 

 the Palaeozoic series. 



Old Red Sandstone and Devonian Rocks — Some of the phenomena 

 connected with the Devonian rocks and Old Red Sandstone are 

 more difficult to unravel with precision than those of Silurian age, 

 for several reasons. 



1st. It is understood to be the opinion of one of our best geolo- 

 gists that in England the true Devonian rocks are the equivalents, 

 in another area, of the Upper Silurian beds themselves ; and this 

 supposition is not weakened by the circumstance that in Cornwall 

 the Devonian beds lie directly on Lower Silurian strata. Against 

 this opinion I think we may urge, with reason, the almost total ab- 

 sence of the species common in the acknowledged Upper Silurian 

 districts in this country ; while in other regions, the fossils of which 

 are almost identical with those in our Devonian beds, the strata 

 in which these fossils occur are found lying above Upper Silurian 

 beds, and notably in North America there is an unconformity 

 between them, which I have seen. 



2nd. Though our Devonian rocks and Old Red Sandstone are 

 both of dates that come between the Upper Silurian and Carboni- 

 ferous epochs, it by no means follows that .they are throughout of 



