Geological Society. 7o 



tion of the absorption is the same for all the liquids belonging to 

 one of the two classes. The action on light of iodine dissolved 

 in alcohol greatly resembles the effect it produces when in the solid 

 state ; whilst the absorption of its solution in carbon bisulphide, 

 and in other liquids of that class, bears, as has been pointed out 

 to me by Professor Stokes, the same relation to the absorption- 

 spectrum of the vapour as the spectrum of the solution of a coloured 

 gas (nitrogen peroxide for example) does to that of the gas. 



GEOLOGICAL SOCIETY. 

 [Continued from vol. ii. p. 548.] 

 November 22, 1876.— Prof. P. Martin Duncan, M.B., P.P.S., 



President, in the Chair. 

 The following communications were read : — 



1. "On the Prc-Cambrian (or Dimetian) Pocks of St. David's." 

 By Henry Hicks, Esq., F.G.S. 



Peferring to the ridge of pre-Cambrian rocks, which he described 

 in a former paper as running down the St. -David's promontory, and 

 as previously supposed to consist of intrusive syenite and felstone, the 

 author stated that he had now found it to be composed exclusively 

 of altered sedimentary rocks of earlier date than the Cambrian de- 

 posits, the conglomerates at the base of which are chiefly made up 

 of pebbles derived from these rocks. Eecent investigations had led 

 him to tho conclusion that the main ridge was composed of two 

 distinct and decidedly unconformable formations, the older of which, 

 composed of quartzites and altered shales and limestones con- 

 stituting the centre of the ridge, has a N.W. and S.E. strike, and 

 dips at a very high angle ; whilst the newer series, consisting of 

 altered shales, and having at its base a conglomerate composed of 

 pebbles of the older rock, has a strike nearly at right angles to that 

 of the latter. For the former he proposed the name of Dimetian, and 

 for the latter that of Pebidian. The author indicated the points of 

 resemblance between these pre-Cambrian Pocks and the Laurentian 

 of Canada, the Malvern Pocks, and others in Scotland and else- 

 where, but thought it safer at present to abstain from attempting 

 any definite correlation of them. The exposure of the older, or 

 Dimetian, series led the author to ascribe to those rocks a thickness 

 of at least 15000 feet : the upper, or Pebidian rocks, which flank both 

 sides of the old ridge through a great portion of its length, are 

 apparently of considerably less thickness ; but they are in most parts 

 more or less concealed by Cambrian deposits overlying them uncon- 

 formably. Punning nearly parallel with Pamsey Sound is another 

 large mass of the author's Pebidian rocks ; and at the south-western 

 extremity of Pamsey Island they compose a bold hill almost 400 

 feet high ; and on the east side of this a fault, with a downthrow of 

 at least 14,000 feet, has brought the Arenig beds into contact with 

 the pre-Cambrian rocks. 



2. " On the Fossil Vertebrates of Spain." By Prof. Salvador 

 Calderon. 



This paper contained a few introductory observations on the 



