TRANSACTIONS OP SECTION C. 429 



glomerata being fairly abundant at Peakshill, and Amplexus coralloides is found 

 at Treak Cliff, but is extremely rare. 



These facts lead to a consideration as to how far the types mentioned are of 

 value in zonal determinations. If any one of them, as recorded from Castleton, 

 be regarded as representing exactly the same form as that recorded from the 

 Bristol area, then its value as one of a number of index fossils of a zone becomes 

 negligible. Examples are cited in the cases of Spiriferina octoplicata and 

 SchizopJioria resupinata. If the Castleton forms of D, horizon agree in identity 

 with the Bristol types of K, and Z, respectively, then these two types become 

 worthless as sub-zon<al indices. It was pointed out that, even allowing of the 

 rather unlikely possibility that in all the cases cited the Castleton specimens 

 represented mutational forms of the Bristol species, the real difficulty as to their 

 zonal value is not overcome, since the line of demarcation between mutations is 

 more or less arbitrary and there is still a considerable field of discussion as to 

 what constitutes a ' mutation.' 



It appears probable that any system of zonal indices can be of local value 

 only, as for example in the application of the Bristol zonal indices within the 

 Bristol area, and cannot be of any general application. 



6. Shift of the Western Shore-Line in England and Wales during the 

 Avonian Period. By Arthur Vaughan, M.A., D.Sc, F.G.S. 



[The terms employed are merely explanatory and are not defended as the 



best that could be chosen.] 



Join Wales across St. George's Channel to Co. Wicklow so as to form an 

 iinbroken mass of land : this mass will be termed 'Wales.' The sea south of 

 this mass will be termed the ' S.W. Channel ' ; it is an. extended and modified 

 Severn Channel. 



.Join Lakeland, Isle of Man, and Co. Down by a much indented shore-line : 

 this is the northern shore of what will be termed the ' N.W. Channel,' the 

 southern shore of which is the north coast of 'Wales.' 



The shore-line under investigation may be considered at any period of the 

 Avonian to be that which delimits a broad central and projecting land-mass 

 ('Wales'), and two more or less narrow branches of the sea {'N.W. Channel' 

 and ' S.W. Channel ') above and below. 



Explanation of Map. 



A pattern indicates the date of the earliest (or latest) deposit at a particular 

 point. In the 'N.W. Channel' the pattern indicates the date of the earliest 

 Avonian deposit at the place. 



In the 'S.W. Channel' the pattern indicates the date of the latest Avonian 

 deposit at the place. 



Hence, shore-lines (continuous patterned-lines) on the north coast of ' Wales ' 

 indicate an adrance of the sea on to the land; whereas, on the south coast of 

 ' Wales ' they indicate retreat of sea and expansion of land. 



The Shore-lines at successive Zonal Dates. 

 [For zones and evidence, consult Report, Brit. Assoc, Winnipeg, Section C, 

 and the later research of several investigators.] 

 i8— Lower Tournaisian. 



'N-W. Channel' scarcely, if at all, defined and confined to a shallow bay 

 m Ireland : ' Wales ' continues northwards in an unbroken mass. 



'S.W. Channel' deeply deflected northward over Hereford to beyond Clee 

 Hill: the most extensive encroachment of the 'S.W. Channel' upon the 

 Wales mass during Avonian time. 



C, — Top of Tournaisian. 



The ' N.W. Channel ' was initiated and Lakeland girdled by its earliest 

 Avonian deposits. 



• T'^^ deepest C, deposits (which much resembled those of Rush) accumulated 

 in the Chtheroe depression. 



