448 Geological Society. 



It has been found that there is a much greater percentage 

 of water given off from the rock at 105° C. in the case of the 

 glassy variety, and the authors suggest that the escape of this 

 excess of water, soon after the consolidation of the rock, has 

 resulted in the devitrification of the sheaths and margins. 



February 24th.— Dr. A. Smith Woodward, F.R.S., President, 

 in the Chair. 



The following communications were read : — 



1. ' The Ashgillian Succession in the Tract to the West of 

 €oniston Lake.' By John Edward Marr, Sc.D., F.R.S., F.G-.S. 



2. ' The Radioactive Methods of Determining Geological Time.' 

 By H. S. Shelton, B.Sc. 



The radioactive 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 com- 

 position 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 radioactive 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 radioactive 

 methods should always be errors of overestimation. The next step 

 in the investigation of the time problem is to be found in a rever- 

 sion to other lines of reasoning. The sea-salt methods, and those 

 based on the thickness of the sedimentary rocks in particular, need, 

 careful reconsideration. Reference is made to a number of papers 

 which show that the first of these is worthless, and the secpnd based 

 on a misapprehension of the nature of deposition. The argument 

 from tidal retardation is still of value, as also is that from the 

 evolution of carbonate of lime. To the author radioactive experi- 

 ments come as a confirmation of views held on other grounds, but 

 are not sufficiently important in themselves to be authoritative 

 against the balance of the evidence derived from other lines of 

 investigation. 



