ELECTROMOTIVE FORCES IX THE VOLTAIC CELL. 529 



The theory of gaseous collision, combined with the density of liquids, 

 suggests a range lying between 7 X 10 -9 and 2 x 10 -9 . 



The dispersion of light seems to require atomic dimensions to lie 

 between 10~ 7 and 1C- 9 . 



The final estimate made by Sir Wm, Thomson is something between 

 2 x 10~ 7 on the one side, and 10 -9 on the other. But if the reasoning in 

 the present paper be admitted as correct, it would seem possible to reduce 

 this range of uncertainty and to make an even more precise estimate. 



On the Archaean Rocks of Great Britain. By Professor T. G. Bonney, 

 D.Sc, LL.D., F.R.S., Pres. G.S., Fellow of St. John's College, 

 Cambridge. 



[A communication ordered by the General Committee to be printed in extenso 



among the Reports.] 



Two methods of dealing with this subject, on which I have been asked 

 by the Organising Committee of Section C to prepare a paper, naturally 

 suggest themselves. The one is to treat it historically, by giving in 

 chronological order & precis of the papers or books in which reference has 

 been made to the Archaean rocks of Britain ; the other to describe, as 

 accurately as is possible in a limited space, the petrology of the several 

 districts, stating briefly at the same time the reasons which have led the 

 writer, in common with many other geologists, to consider their rocks as 

 more ancient than the Cambrian period. 



The former method has doubtless many advantages, and would be the 

 fit one, had I been called upon to ' report ' on the Archasan question ; 

 but it does not commend itself to me, in this particular case, as the one 

 most likely to be helpful to those who are more especially engaged in 

 petrological studies. Speaking for myself, I always find it, in the case of 

 a district not yet visited, more useful to be informed as exactly as 

 may be what are the mineral and physical characters of its rocks, and 

 what their stratigraphical relations, than what opinions have been enter- 

 tained as to their antiquity. For this purpose it is not enough to have 

 them named, unless the grounds of the writer's nomenclature are given. 

 There has hitherto been so much latitude assumed in the use of such terms 

 as ' schist ' and ' slate,' not to mention others, that they become of little 

 value unless what we may term the ' personal equation ' of the writer 

 be known. Accordingly, in this paper, I shall endeavour to give as exactly 

 as is possible, without entering into minute details, the mineral characters 

 of the Archaean rocks in each district noticed, and the relations in which 

 they stand to those of ascertained geologic age. I may add that 

 throughout I shall use the term ' schist ' to denote a more or less 

 foliated rock — that is, one in which presumably great mineral changes have 

 taken place since its materials were first deposited, so that, if they were 

 originally clastic, few, if any, traces of the constituent grains can be recog- 

 nised; ' schistose,' to denote that a rock looks like a schist, but in this 

 case I imply no more than an external resemblance. By the term meta- 

 morphic rocks I mean those which, like schists, have undergone great 

 mineral changes — not rocks which, like many which are schistose, have 

 been in reality but slightly altered, whose changes have been only micro- 

 mineralogical. The word ' slate ' is never applied to a foliated rock, 



1884. ii m 



