ON THE ARCILEAN BOCKS OF GREAT BRITAIN. 551 



Maree and Loch Shiel series of Dr. Hicks. Concerning the Upper Lauren- 

 tian or Norian series I am not yet prepared to offer an ojjinion, I believe, 

 indeed, that much of it is igneous rock, and so may not avail for pur- 

 poses of comparison at so great a distance. The other members that I 

 have seeD do not recall to my memory any of the Scotch rocks. It is 

 needless to say that the gabbros of Skye are igneous in origin, and much 

 more modern than the norites of the Upper Laurentian. 



The Gairloch and Ben Fyn series has a very considerable resemblance 

 to a suite of specimens from the White Mountains shown to me by Dr. 

 Sterry Hunt, and forming the sei'ies called by him Montalban. These, so 

 far as I have seen, are not generally well represented in Canada, but I 

 think it very probable that it will be found necessary to remove from the 

 Huronian certain schists now provisionally placed at its base, and regard 

 these as representatives either of the Montalban group (which litho- 

 logically is represented by the great zone of Alpine schists at the Val 

 Tremola, St. Gothard) or of the immediately overlying zone of schists 

 (schistes lustrees, Bundner schiefer in part, &c), concerning whose age in 

 the Alps such diverse opinions have been entertained. To this group 

 the newer schists of the Highlands are probably related, and some at 

 least of the schists of Anglesey and of the south of England — though it 

 is possible the metamorphic rocks of Cornwall may be older. 



Some beautiful reddish felstones shown to me by Dr. Sterry Hunt, 

 members of the Petrosilex group, are singularly like the felstones at the 

 base of the Pebidian of Wales (Arvonian of Dr. Hicks). The great 

 mass of the Canadian Huronian, of which I have now seen many specimens, 

 and examined carefully along a section of not a few miles, corresponds 

 rather closely with the great series of slates", grits, agglomerates, and lavas 

 which immediately underlies the Cambrian — viz., the Pebidian (with 

 the Arvonian of Dr. Hicks). The Huronian is not a highly altered series. 

 I feel certain that the microscope will generally disclose its original con- 

 stituents. I suspect, then, that in many parts of Canada there is an 

 enormous break between the Laurentian and Huronian, though here and 

 there this may be partially bridged over. Further, if we are to put any 

 trust in lithological chai*acter as a test of relative age, I hold it impossible 

 to consider the Huronian older than the Montalban, or to correlate it 

 with the Pietra Verde group of the Alps. I have made no reference to 

 the Taconic group of America, because I have not had the opportunity 

 of studying it either in the museum or in the field. 



On the concordance of the Mollusca inhabiting both sides of 

 the North Atlantic and the intermediate Seas. By J. Gtwyn 

 Jeffreys, LL.D., F.R.S. 



[A communication ordered by the General Committee to be printed in exte wo among 



the Keports.] 



Doring a short but delightful and interesting visit to North America 

 which I made in the summer of 1871, on the invitation of the lamented 

 Professor Agassiz, for the purpose of examining the Mollusca procured 

 by the late Count Pourtales in the Gnlf of Mexico, I took the opportunity 

 of inspecting also many other collections of North American Mollusca, 



