﻿lxXXvi PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I905, 



Keisley Limestone, 1 I quite agree with him that ' its paheonto- 

 logical features point to its stratigraphical position being at the 

 base of the Upper Bala ' (that is, Ashgillian). 



As regards the faunas of the Ashgillian Series, the basal lime- 

 stone seems everywhere to contain Staurocephalus, though not 

 always in abundance. It has accordingly been referred to in 

 Britain and Scandinavia as the ' Staui*oce-p7ialus-Limestone.' > The 

 shales above usually yield Dalmannites mucronatus, a form which 

 passes into the Llandovery rocks of the Silurian system. 



The Ashgillian fauna of Britain requires to be fully worked out. 

 Its forms are found in profusion in the Bala district, in the region 

 around Haverfordwest, in the Lake District and the Cross-Pell 

 inlier, and probably in the Girvan district of Scotland and in the 

 North of Ireland. 



My remarks on the classification of the Ordovician strata are 

 designed to show that, as our knowledge increases, more accurately- 

 defined planes than those which were used for purposes of classi- 

 fication by the pioneers of stratigraphical geology are necessary. 

 This involves a certain amount of change of nomenclature, but 

 I have endeavoured to show that such change may be made 

 without any violent alteration of the nomenclature which is at 

 present in use. 



We have time-names for the rocks of most of the great systems — 

 as, for instance, the Olenellian, Paradoxidian, Olenidian, and 

 Tremadocian in the Cambrian System ; the Valentian, Salopian, 

 and Downtonian in the Silurian ; and the Taunusian, Coblenzian, 

 Eifelian, and Clymenian in the Devonian. As our knowledge 

 increases, we shall refer the beds of new areas to their places 

 among these different series, marking periods of time, with a 

 confidence similar to that with which we have long assigned strata 

 of remote regions to one or other of the great systems. 



1 ' The Fauna of the Keisley Limestone : — Part II ' Quart. Journ. Geol. 

 ■Soc. vol. liii (1897) p. 67. 



