17-i THE YALEyTTAX SEBIES. [vol. Ixxvii. 



possible to attribute these differences to variation within that 

 formation, although this may not be the true explanation. 



The resemblance of this group in lithological characters and 

 mode of weathering to the much thinner band which occurs at the 

 base of the Silurian in the Lake District and Moifat, and its 

 position with respect to the Acuminatus Zone, renders it probable 

 that the zone-fossil may yet be found in the very lowest part of 

 the Silurian in the northern districts. Its common associate, 

 Climacograpius normaJis. preserved as in Central Wales in high 

 relief, has already been noted in that position. It is satisfactory 

 to find that the zone-fossil occurs in a shale-band between the grits 

 of Grwastaden near Khayader, not many feet above their base. 

 The horizon adopted by Dr, Herbert LapAvorth on lithological 

 grounds agrees, therefore, with that proposed above for faunal and 

 physical reasons. 



It is probable that the abrupt lithological change at the base of 

 the Silurian in all areas is the result of a change in the physical 

 conditions of deposition, which appears to have brought about a 

 marked palseontological break. The mottling of the basement- 

 beds, which is due to the inclusion of clots and flecks of a dark 

 mudstone among the pale sediments, and the high content of 

 pyrite are suggestive of arrested sednnentation, if not, also, of 

 actual erosion. 



Discrssiox. 



Dr. H. Lapwoeth congratulated the Author on his paper, and on 

 the fresh data that he had brought forward regarding this puzzling- 

 series. From his own knowledge the speaker agreed that the name 

 ' Tarannon ' was vague, but so was ' Llandovery ' ; and he felt that, 

 until further research was earned out in the Llandovery district 

 itself, and more detail had been secm-ed from the Corw^en-Cerig- 

 Y-druidion area, anv restriction in nomenclature might lead to 

 controversy. The speaker's own view of the Yalentian was that 

 deposition appeared everywhere in Britain to be continuous from 

 the Persculptiis Zone, or base of the Silurian, up to at least the 

 Convolutus Zone. At or immediately above this horizon there 

 m.ay or may not have been local elevation and a break for a time ; 

 but great depression apparently continued for the remainder of the 

 Yalentian Period, so that one might find any of the succeeding 

 zones as the base of the series resting upon older rocks. 



The Author thanked the Fellows for their kind reception of 

 the paper, and in reply to the previous speaker stated that an 

 account of the succession in the Llandovery area Avas included in 

 the paper. Although the structure of that area -was still imper- 

 fectly known, sufficient was knoAvn of its fauna to make a correla- 

 tion with other districts possible. The Author hoped at an early 

 date to resume the mapping of that district, which had been 

 interrupted by various circumstances for many years. 



[August 19th, 1921.] 



