Ch. XXVII] 



LOWER CAMBRIAN. 



449 



often be somewhat arbitrary, yet in no part of the world have we 

 hitherto examined a succession of rocks having so great a thickness as 

 45,000 feet, even where they are made up in part of volcanic materials, 

 which have been referred to one period as being characterized by one and 

 the same fauna. 



The first formation mentioned by Prof. Sedgwick, beneath the Bala 

 Limestone (and its associated beds of sandstone) in JT. Wales, are certain 

 beds, 7000 feet thick, called the Arenig slates and porphyry. Under 

 them he finds theTremadoc Slates, 1000 feet thick, and next the Lingula 

 Flags, already described, 1500 feet or more, which, in accordance with 

 views first put forward by Mr. Salter, I have referred provisionally to an 

 Upper Cambrian group. . ; 



Lower Cambrian. — To the Lingula Flags last enumerated, another 

 series, called by Prof. Sedgwick the Bangor Group, succeeds in the de- 

 scending order, comprising, first, the Harlech Grits, 500 feet thick, and 

 next the Llanberis Slates, 1000 feet. These formations have as yet proved 

 barren of organic remains in N. Wales; but in Ireland, immediately 

 opposite Anglesea and Caernarvon, rocks of the same mineral character 

 as the Bangor Group, and occupying precisely the same place in the 

 geological series, have afforded two species of zoophytes, to which Pro- 

 fessor Forbes has given the name of Oldhamia (figs. 611 and 612). The 

 position of these rocks has been decided by the Government Surveyors, 



The most Ancient Fossils yet known (1854). 



Fig. 611. 



Fig. 612. 



Oldhamia radiata, Forbes. 

 Wicklow, Ireland. 



Oldhamia antiqua, Forbes. 

 Wicklow, Ireland. 



and confirmed by Sir R. Murchison, so that here we behold the relics of 

 the most ancient organic bodies yet known. We are of course unable at 

 present to determine whether they belong to the same fauna as the fossils 

 of the " Lingula Flags," or to an older one. The beds containing them 

 may provisionally be called Lower Cambrian, for it will always happen 

 that our inquiries will terminate downwards in rocks affording very im- 

 perfect materials for classification. This will continue to be the oase, 

 however many steps we may make in future in penetrating into the re- 

 moter annals of the past. 



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