76 SECTIONAL ADDRESSES. 



The Yoredale rocks of Wensleydale include typical algal ' lagooii- 

 phase ' deposits intercalated with the standard limestone, shale and 

 sandstone, but the finest development of such deposits in the north of 

 England hitherto described is that of the Solenopora sub-zone of Garwood 

 at Shap and Raveustonedale. 



In the Winnipeg report it might appear that each of the three facies — 

 standard, Zaphrentid and Culm — is to be regarded as a phasal equivalent. 

 Dixon,^^ however, points out that this was not quite the sense in which 

 Vaughan intended the expression to be used. He did not regard the 

 standard limestones as a phase, but applied the term only to the 

 Zaphrentid and Culm developments. I can see no reason why the 

 calcareous development, even though recognised as ' standard ' in 

 Vaughan's sense, is not to be regarded as a phase exactly as with the 

 other phases. 



D., is the horizon of greatest importance from the point of view of these 

 phasal equivalents. The symbol D.^ was first used by Sibly for rocks in 

 the Midland area overlying D.^. These rocks are of Zaphrentid-phase 

 type, and it has hence been frequently assumed that Sibly intended to 

 restrict the symbol D., to rocks of this character. I have never been 

 able to see that there was anything phasal in his original use of the symbol, 

 and he informs me by letter that the symbol was, as I imagined, originally 

 employed in a chronological, not phasal, sense. This usage was not, 

 however, adhered to. Vaughan, while originally iising the symbol in a 

 chronological sense, subsequently, as in the Loughshinny paper, employed 

 it in a phasal sense as indicating rocks of Zaphrentid phase at any level 

 in the D beds. He employed the symbol Dy in a chronological sense as 

 equivalent to Dj and D^. Other authors have followed Vaughan in using 

 D., in a phasal sense. Thus the symbol is attached to the Botany Beds 

 in Garwood's vertical section of the succession in the North-west Province, 

 and clearly indicates a phasal use of the term. 



The net result of this varying use of these symbols has proved very 

 confusing, and probably no worker would disagree with Sibly and the 

 British Association Committee in recommending that the use of Dy in 

 a chronological sense be discontinued. Sibly,^^ when discussing the 

 nomenclature of the D beds, suggested that, taking the numbers 1, 2, 3 to 

 indicate time divisions, the letters y,x and p should be added to indicate re- 

 spectively the standard or calcareous phase, the Zaphreutid-Cyathaxonid 

 phase, and the Culm or lamellibranch-goniatite phase. If this method 

 were adopted and were extended to the whole Avonian system it would 

 afford a simple method of stating the character of the rocks at any level or 

 locality, and would be specially useful in the case of developments like 

 those of Loughshinny, Gower and Wensleydale, where more than one 

 phase is represented at a horizon ; thus at Gower lower D , is D.,x, upper 

 is D.,p. If all phases were dealt with in this way any suggestion that one 

 phase was less important than another might be avoided. 



The foregoing account is chiefly confined to the phases of deposition 

 originally recognised by Vaughan. There are, however, other well- 



33 Geol. Mag., vol. Ixii. (1925), p. 382. 

 31 Proc. Geol. A.ss., vol. xsxi. (1920), p. 81. 



