Dr. H. Eiclis — Life Zones in Palceozoic Jtocks. 401 



and for many years afterwards they remained the only organisms 

 known in the Lower Cambrian rocks of Britain. 



In the year 1881 I decided to divide the Lower Cambrian into 

 three groups ' under the names from below upwards of Caerfai, Solva, 

 Menevian. I pointed out that it had a " natural base made up of 

 massive conglomerates and sandstones," and that "its upper boundary 

 is well defined by an important paltBontological break," and that it is 

 " clearly divisible at St, David's, where it has chiefly been explored 

 of late years into three groups." 



In speaking of the Caerfai Group I said that " beds of the same age 

 are also found in the Harlech Mountains. The great slate quarries 

 of Llanberis, Bethesda, and other places in Carnarvonshire are also 

 on this horizon. Though the basal beds are not exposed in the 

 Longmynd area, it is probable also that some belonging to this 

 group occur there. There is a strong general resemblance between 

 the beds of this group in each of the areas, but as they were at 

 first shore deposits around a subsiding land-area, some differences in 

 appearance and in thickness must necessarily occur." 



•In the year 1887 Dr. Woodward^ described a Conocoryplie {C. 

 viola), specimens of which had been found in the Upper Green 

 Slates of the Penrhyn Quarries by Prof. Dobbie, and Messrs. K. E. 

 Jones and R, Lloyd (two quarrymen employed in the Penrhyn 

 Quarry, Bethesda, Carnarvonshire). This very important discovery 

 of fossils in the undoubted Cambrian of the Geological Survey in 

 North Wales, was hailed with delight by those who had been 

 working in the Cambrian rocks in other areas, and it was hoped 

 that additional fossils which would enable the horizon to be definitely 

 fixed would soon be discovered. Up to the present, however, only 

 one or two additional forms have been found in the quarrj^ and the 

 horizon has mainly to be fixed by stratigraphical evidence. The 

 ConocorypJie is undoubtedly of the type restricted to zones low down 

 in the Cambrian ; and, in my opinion, until further evidence is 

 obtained, it may fairly be placed at as low an horizon as the base 

 of the Solva Group of St. David's. 



The stratigraphical sequence here, I described briefly as follows 

 in the Eeport of an Excursion of the Geologists' Association to 

 Penrhyn Slate Quarry and Nant Ffrancon in 1883 (vol. viii. No. 4) : — 

 " In traversing this section from west to east, along the sides of 

 Nant Ffrancon, the following general order of the rocks seemed 

 tolerably clear : — 



1. The purple and green slates of the quarry (Llanberis). 



2. A thick series of grits and flaggy sandstones (Harlech). 



3. Dark flaggy beds (Menevian). 



4. Flags and sandstones with Cruziana (Lingula flags). 



5. Dark flags (Tremadoc?). 



6. Black slates and flags with pisolitic iron ore and grit at base 

 (Arenig). 



^ ' ' The Classification of the Eozoic and Lower Palaeozoic Rocks of the British 

 Isles," Popular Science Review, 1881, and Proc. Geol. Assoc, vol. viii. No. 5, 1881. 

 ^ Q.J.G.S., vol. xliv. p. 74. 



DECADE IV. VOL. I. ^NO. IX. 26 



