Notices respecting New Books. 317 



respects, the reverse of what obtains in most parts of the area west 

 of the Severn, where a confused and irregular unconformity, or series 

 of unconformities, reigns about the horizon represented by the 

 Llandoveries. 



2. Absence of important break in Sedgwick's Cambrian. — The 

 Sedgwickian, desirous of establishing the systematic unity of all 

 the Palaeozoic rocks below the base of the May Hill, is bound to 

 minimize the gaps and changes occurring between the Bala, Llan- 

 deilo, Arenig, Tremadoc, Linguia Flags, Menevian, and Harlech, 

 which he alleges were, with the exception of the last, swallowed 

 one after another by the voracious Murchison. Hence Mr. Marr, 

 whilst approving of the strong line drawn between the Silurian and 

 Ordovician (i. e. between the Upper and Lower Silurian), affirms 

 that Prof. Lapworth has drawn " another line between Ordovician 

 and Cambrian at a position where there is no break." This is at 

 the base of the Lower Arenig. Substantially we presume that this 

 line coincides with the one adopted by Lyell and Hicks in 1871. 

 Whether right or wrong, it is at least important to note that the 

 two men, who have done more than any others to work out the 

 obscure and difficult palasonWogy of the Lower Palaeozoics, viz. 

 Hicks and Lapworth, are in substantial agreemert as to where a 

 sharp line should be drawn, and they therefore restrict the term 

 Cambrian to the series below the base of the Arenig. Mr. Marr, 

 however, finds that in the lists of the Tremadoc fossils given in the 

 Cambridge catalogue nearly 60 per cent, of the genera range into 

 the Arenig rocks, and not 20 per cent, into the Ffestiniog or Linguia 

 Flags ; and he further quotes Mr. Etheridge (Mem. Greol. Survey) 

 to the effect that, out of 85 Tremadoc species recorded, 15 pass 

 upwards into the Arenig rocks, and only 8 downwards into the 

 Linguia Flags. 



Surely this is a little unfair of Mr. Marr after his general 

 disclaimer of the method of percentages ! He seems inclined to use 

 this method when it suits his purpose, and we are thus forced to 

 the conclusion that if it told in his favour he would not hesitate to 

 adopt it elsewhere. Let us see how Mr. Etheridge explains the 

 fact in his Presidential Address (Quart. Journ. Geol. Soc. vol. xxxvii. 

 p. 98). " More than 50 per cent, of the species pass (from the 

 Tremadoc) to the Arenig, or 11 genera and 16 species. These few 

 species little affect the question of the palaBontological break that 

 takes place here : for 40 new genera and 133 new species make 

 their appearance in the succeeding Arenig, through some physical 

 changes accompanying the zoological, which we have not yet been 

 able satisfactorily to discover. Unconformity between this group 

 and the Arenig is not known in the typical areas of South and 

 North Wales ; but at no time in the history of the lowest Palaeozoic 

 rocks has apparent extinction on the one hand, and migration from 

 some unknown area on the other, taken place so markedly." 

 Hence the late President of the Geological Society is clearly in 

 accord with Messrs. Hicks and Lapworth as to the magnitude of the 

 palaeontologieal break at the base of the Arenig. 



