354 On a Ternary Geological Classijieaiion. [July, 



strata of the Carboniferous rocks of Britain.* I there remarked 

 that " we cannot fail to have observed that many groups of strata 

 have a tendency to arrange themselves into threefold divisions, 

 the upper and lower being composed of sandstones and shales, the 

 middle of limestone." 



As the subject was foreign to the question before me at the 

 time, except as bearing on the Carboniferous series, I did not pursue 

 it ; but in a paper more recently read before the Geological Society 

 of Glasgow (in 1867), and piiblished in then* Transactions,! on 

 another branch of the subject, I entered rather more fully into 

 the discussion of this question, and ventured to state certain general 

 principles as bearing on the operations of nature dm-ing past 

 geological periods, which I venture to repeat here — 



1st. That in any formation composed of contemporaneous 

 calcareous and sedimentary strata, these two classes of rod's must 

 have had their sources in ojJjwsite centres or lines of distribution. 



2nd. That in the same formation, consisting of calcareous aiid 

 sedimentary materials, the maximum development of each class has 

 been reached in positions relatively opposite to each other. 



In that paper I also proposed a thi'eefold arrangement of the 

 British formations from the Upper Silurian to the Tertiary inclusive. 

 Since then I find that somewhat similar views have forced them- 

 selves on the mind of a distinguished geologist, Dr. Dawson, F.li.S., 

 of Montreal, who, in his elaborate paper on " The Conditions of 

 Distribution of Coal as illustrated by the Coal-formation of Xova 

 Scotia," arranges the Carboniferous series of that countiy under 

 three heads corresponding to those of Great Britain, and formed 

 under similar j^hysical conditions. 



These alternations lead Dr. Dawson to the suggestion of 

 " Geological Cycles " for the Palaeozoic rocks of America, but he 

 adopts a fourfold classification, though, as it seems to me, a three- 

 fold is a more natural one, at least as regards the Carboniferous 

 group. The following is Dr. Dawson's quaternary classification, 

 which he has more recently incorporated in his work, 'Acadian 

 Geology :' t— 



* Vol. xviii., p. 134. t Vol. iii., pnrt 1. 



% Altliougli l)r. Dawson's pnper was published nioro ivccutly than my own 

 already alluclcd to, it seems to have escaped liis ol)scrvation. This faet is sjitis- 

 faetory, as showing that on opposite sides of tiie Atlantic tliore is evidence ot a 

 natural order of ariangement aceonh'iig to geological cycles, and Indicated by the 

 miuenil character of the strata. Witli regard, liowever, to the arrangement j)ro- 

 posed liy Dr. Dawson, it ajjpears to me that a tiireefoM classilieation might equally 

 well be adopted, at least in the ease of the Carboniferous series, and that there aro 

 no good grounds for dissevering the third and fourth stages. In tlie case of tiio 

 other groups, it is a question for further inquiry whether the series is not also 

 capable of arrangement according to a ternary systcMii ; but in order to determine 

 this i)oint, a fuller knowledge tiian I possess of the relative importance of the 

 several nieinlieis of the series, and their relations to each other over the whole 

 area would be required. 



