32 



Featherstonhaugh''s Geological Report, 



We now approach a system of rocks usually found overlying 

 the inferior division, and where organic remains are for the first 

 time found in the ascending series. Perhaps the formation 

 alluded to as clay slate belongs properly to this system, 

 the general mineral structure of which is altogether slaty. 

 The whole of this series of rocks (a luminous account of 

 which will soon be published by the Rev. Adam Sedgwick, 

 under the designation of the Cambrian system, from the 

 locality where he has principally studied them) has beea 

 hitherto included, together with the Silurian system of Mr. 

 Murchison, under the undefined names of transition and grau- 

 w^acke, terms which are now likely to be entirely abandoned. 

 Before this portion of the geological column had been ade- 

 quately investigated, the theoretical term transition was gen- 

 erally received. It was supposed that at the point where 

 organic remains first appeared, there existed a ti'ue natural 

 mineral transition from inorganic to organic rocks ; and if that 

 point were established, the term would not be misapplied to 

 those slaty masses. But the affinity of the mineral structure of 

 some of those rocks with that of others higher up in the series 

 caused the term to be extended so far beyond its original mean- 

 ing and application, as to comprehend deposites of from thirty to 

 forty thousand feet in thickness ; and although this great slaty 

 system was separated from the carboniferous limestone by the 

 old red sandstone formation, itself having a thickness of ten 

 thousand feet and more in many localities, yet some geologists 

 included all these formations, together with the carboniferous 

 limestone, containing a distinct class of organic remains, in 

 the transition rocks. Henceforward, it is probable that the 

 term will fall into disuse, in proportion as the nomenclatures 

 applied to the Cambrian and Silurian systems shall become 

 generally known, they having already received the sanction 

 of the leading European geologists. 



The loiver Cambrian of the tabular view, as it has been 

 observed in Caernarvonshire, .consists principally of chlorite 



