Reports & Proceedings — Geological Society of London. 187 



2. February 24, 1915.— Dr. A. Smith Woodward, P.R.S., President, 



in the Chair. 



The following communications were read : — 



1. " The Ashgillian Succession in the Tract to the West of Coniston 

 Lake." By John Edward Marr, Sc.D., F.R.S., F.G.S. 



The author has studied in detail the succession of the Ashgillian 

 strata in Ashgill Beck and the adjoining tract. In Ashgill Beck the 

 following sequence was detected : — 



Valentian. 



Thickness in feet. 

 rUpper Ashgill Shales . about 50 



} (Phacops inner onatus beds , 16 



Ashgillian -J Middled Ash 16 



s [White Limestone . about 12 



I Lower Phillipsinella beds ... 7 



Caeadocian. 



An account of the lithological characters and lists of the fossil 

 contents of the various divisions are given, and confirmatory sections 

 from Coniston Village to Appletreeworth Beck are described. 

 A comparison is made with the beds of the Cautley district, previously 

 described by the author. Some fossils which have not yet been found 

 in the Lower Ashgillian of the Cautley district occur in the beds of 

 that division at Coniston. 



Prom a study of the fossils of the Coniston tract and of other areas 

 in Britain and the Continent, it would appear that a twofold division 

 of the Ashgillian strata which is of more than local value may be 

 made. The lower division is characterized by the abundance of 

 Phillipsinella parabola, and the upper by the profusion of Phacops 

 mucronatus. 



2. " The Radio-active Methods of Determining Geological Time." 

 By H. S. Shelton, B.Sc. (Communicated by Professor W. J. Sollas, 

 Sc.D., F.R.S., P.G.S.) 



The radio-active method of determining geological time, while of 

 great interest, is not of such certainty as to be independent of con- 

 firmation from other lines of investigation. The various radio-active 

 methods, helium ratios, lead ratios, and pleochroic haloes are severally 

 examined, and the various sources of uncertainty, general and 

 particular, are pointed out. The most important general cause 

 of uncertainty is to be found in the fact that mechanical and chemical 

 changes of composition in minerals are the rule rather than the 

 exception ; and, in instances where constancy of composition 

 throughout long periods of geological time is asserted, the burden of 

 proof lies with those who make the assumption. The attempt to 

 assess exact or even approximate times by' means of lead ratios is 

 premature and entirely invalid. At the same time, the weight of the 

 evidence is such as to render it exceedingly probable, so far as 

 radio-active evidence goes, that geological time must be reckoned at 

 least in hundreds of millions of years. There is a high degree of 

 improbability that the errors in the radio-active methods should always 

 be errors of overestimation. The next step in the investigation of 



