158 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 16, 



there is a corresponding difference in the mineralogical development 

 of these corresponding groups. What is here stated must therefore be 

 regarded as provisional, and I doubt not that on a closer comparison 

 a further subdivision of this part of the series will be hereafter made. 

 So far as I know, from my present list of fossils, Cephalopods first 

 appear in this group. It is eminently characterized by Trilobites ; 

 among which Ogygia Buchii (the most abundant fossil of the 

 Llandeilo flag) has not yet been found. It might be called the 

 Upper Cambrian slate group, or the Trilobite group, or (geographi- 

 cally) the Bala group. 



4. Lastly, the Cambro- Silurian group. — In this are included the 

 lower fossiliferous rocks east of the Berwyns between the Dee and 

 the Severn — the Caradoc sandstone of the typical country of Si- 

 luria — and the Llandeilo flag of South Wales, along with certain 

 associated slates, flags, grits and conglomerates, above noticed. In 

 the group thus defined the fossils are in infinite abundance, and are 

 now well known (as a group) since the publication of the " Silurian 

 System," of which they form the palaeontological base. Among 

 these fossils are found numerous and very characteristic species of 

 the Upper Silurian rocks (Wenlock shale, &c.); but considered as 

 a whole, their afiinities connect them more nearly with the fossils of 

 the lower groups of North Wales (above enumerated) than with 

 those of the Upper Silurian groups (Wenlock and Ludlow). Hence 

 it must be considered as an error (now admitted on all hands) to 

 have arranged these Cambro-Silurian rocks with the Upper Silurian 

 groups (Wenlock and Ludlow); and, at the same time, to have cut 

 them off' from the lower formations above enumerated, which occupy 

 the largest portion of North and South Wales, are of enormous 

 thickness, and form a most characteristic physical system, or collec- 

 tive group, at the base of the whole palasozoic series. But incom- 

 parably greater would be the error to regard the Lower Silurian 

 rocks (Caradoc and Llandeilo groups) as the equivalents of all the 

 lower groups above enumerated — and, under this erroneous hypo- 

 thesis, to designate all these lower groups (down to the Lingula beds 

 inclusive) as the porphyrized equivalents or representatives of the 

 Lower Silurian rocks — and hence to designate the vast series of rocks 

 in North and South Wales under the name either of the Lower 

 Silurian system, the Lower Silurian division, or any other analogous 

 term — a designation which is not true to the facts of development, 

 and is in antagonism with all analogy to the language by which our 

 successive palaeozoic deposits have hitherto been defined. As a 

 matter of fact, the group under notice is a group of transition or 

 passage (implied in the name Cambro-Silurian), Its upper beds 

 blend themselves with the Wenlock shale and Upper Silurian rocks, 

 its lower beds pass into the old slate rocks of North and South 

 Wales, to which the collective name Cambrian has been given — and 

 given correctly, whether we regard palaeontological development or 

 geographical propriety of language. 



