504 G. H. Kinahan — Archcean Rocks. 



rocks, or even granite. Of course rocks that have once been metamor- 

 j)hosed, may again be so placed that they are subjected to a second, 

 third, or even more periods of action ; each, if not too excessive, further 

 developing the foliation. The American rocks called Korians or 

 Labradorians have excessively coarse foliation, some of the measured 

 plates being at least a foot in length ; these rocks at one time were 

 considered on account of their structure, to be the oldest Archasan : 

 but further research has demonstrated that they are as young, if 

 not younger, than the associated rocks : as they originally were 

 protrudes or intrudes, generally the latter, possibly of granite, 

 into their associated rocks, and that their present structure is 

 solely due to metamorphic action at successive times. In Ireland 

 intruded courses and tracts of granite in the Castlebar District, Co. 

 Mayo, and near Glenties, Co. Donegal, have been altered into a very 

 coarse gneiss by metamorphic action, while in Slieve Croob, Co. 

 Down, the margin of the granite intrude has also by a similar process 

 been changed into gneiss. It therefore appears evident that if any 

 kind of rock, by sufficient metamorphic action, can be changed into 

 crystalline foliated rock, such structure or composition by themselves 

 cannot be taken as a test of the age of the rocks. 



Dana, Le Conte, Selwyn, George Dawson, and other American 

 authorities specially point out that between the American Archaean 

 and the later rocks there are the distinct records of a vast lapse of 

 time. In the Dominions this is most conspicuous, the unaltered 

 fossiliferous Primordial or Camhriam rocks lying unconformably 

 on the Laurentian gneiss, and the Huronian schists ; the old rocks 

 Laving been tilted, crushed up, metamorphosed and afterwards 

 excessively denuded, prior to the Primordial or other later rocks 

 having been deposited on them. 



In Europe alone there are in different places representatives of 

 the different Passage Beds between the different geological groups ; 

 there are, as Passage Beds between the Cambrian and the Ordovicians, 

 the Arenig group; between the Ordovicians and the Silurians, there 

 are the Llandovery or Mayhill Sandstone group ; between the Silurians 

 and Carboniferous, there are the Devonians or Old Eed Sandstone, and 

 so on upwards ; it is therefore natural to suppose, that somewhere on 

 the face of the globe there are rocks that represent the equivalents of 

 the Passage Eocks between the Arch^an and the Primordial ; but if 

 such rocks occur in England or Ireland, which to me appears very 

 questionable, they are not the equivalents of any of the American 

 Archgeans, and ought not to be so called, but should be grouped as 

 " Passage Beds " under a new and distinct name. 



Except in Scotland, where it has been demonstrated by Murchison, 

 there are no authentic records of a great lapse of time between the 

 so-called ArcliEeans and the later rocks. Circumstances have prevented 

 the writer from being intimately acquainted with the English tracts 

 now declared to be of Archaean age ; but he has visited some of them, 

 and in fact believes he was the first to suggest that the rocks in one 

 of them might possibly be Archaean. Now, however, since the real 

 Arch^an rocks have been seen and studied, such a suggestion in refer- 



