540 KEY. J. F. BLAKE 01^ THE 



intended, like Ms " Uriconian/' to be used as appellatives. More- 

 over, all the preceding detailed descriptions have gone to prove that 

 there is but one great series, of vrhich the terms gneissic and slaty 

 are only partially descriptive of integral parts. 



But we have the loug-established and better-known names of Dr. 

 Hicks : — Dimetian, Arvonian, Pebidian ; the latter name, at least, as 

 that of a well-marked Pre-Cambrian group, has attained a wide circu- 

 lation. Where, then, is the objection to the use of these terms ? 



They are intended by their author to represent systems equi- 

 valent to the Silurian or Cambrian on the one hand, and to the 

 Lewisian or Laurentian on the other. As such, they stand in llr. 

 Etheridge's Presidential Address for 1881, and in Dr. Hicks's popular 

 article in the same year in the ' Popular Science Review.' But what 

 are they actually ? The Dimetian is an intrusive granite, occupying 

 a very small area anywhere. The Arvonian is a portion of the con- 

 temporaneous volcanic rocks of more than one epoch, some Cambrian 

 and some Pre-Cambrian ; and there is only left the Pebidian, which 

 is the remainder of the volcanic products and associated deposits of 

 Pre-Cambrian age. In the whole series of connected rocks it repre- 

 sents only Xos. 5 and 6 of the preceding list. Hence the only name 

 that it would be possible to use covers only a small portion of the 

 series, and has no pretension really to designate a " System." The 

 only way would be to enlarge its meaning and to make it cover the 

 whole of the system of rocks of which it forms a part. I think this 

 plan would lead to great confusion. The name was established, and 

 has been continued to be used as the antithesis to the Dimetian. 

 It could not lead to clearness to use the same term to cover the 

 Dimetian, the Arvonian, and much more besides. It might be re- 

 tained as the name of a minor subdivision, like the Bala among the 

 Ordovicians, or the Corallian among the Jurassics ; but it cannot be 

 used as a name for the whole. 



Then there are the American names, Huronian, AEontalban, I^orian, 

 and I know not what besides. To adopt one of these would be to 

 beg the question of correlation, which has not as yet been so much 

 as seriously attempted. AYe may hope, perhaps, to trace a group of 

 rocks across Europe, but to fix where they will come in America 

 beforehand would be fatal to progress. The same reason would apply 

 to the adoption of any local European name, used for rocks whose 

 relations have not been clearly made out, such as Hercynian. 



There is no alternative left but to propose a new name which 

 shall at once represent the whole group, be taken from the locality 

 where its relations to other groups have been worked out, and shall 

 have some affinity to the names already given to these other groups. 

 As, then, from Siluria we pass downwards geologically and westwards 

 geographically to come to Ordovicia, and from Ordovicia we pass 

 similarly downwards and westwards to arrive at Cambria, so pass- 

 ing on still downwards and westwards from Cambria we come to 

 the Isle of Mona, and here we find rocks developed which we may 

 suitably group together as the Moxian System *. 



* See Brit. Assoc. Eeports, vol, Ivi. p 669. 



