470 Proceedings of the Royal Irish Academy. 



XXII. — Metamoephic Action. By G. H. Kinahan, M.E.I. A., &c. 



[Read, February 23, 1885.] 



PoR years it has seemed to me that certain phenomena in connexion 

 with the metamorphism of rocks have not met with the consideration 

 due to them. In deference to my fellow-labourers in America, but 

 especially in the Dominions, I have not hitherto put these forward ; 

 but now, after having seen the Canadian Laurentians, I venture to do 

 so. 



By way of introduction, it should be mentioned that metamorphic 

 action may be generally placed under three classes^ : — 



Regioncd (metapepsis). — Change due to action invading more or 

 less considerable areas. Its exact genesis is at present somewhat 

 obscure. Some suggest that it is due to aqueo-igneous action, or heat 

 in the presence of moisture, generated by a superincumbent mass of 

 accumulations. According to some chemists, if the time be sufficiently 

 long, the change may take place under the given conditions without 

 the intensity of the heat being at any time great. 



Contact (paroptesis). — This action invades restricted, generally 

 small, areas or tracts, in immediate contact with intrudes of eruptive 

 rocks or vents for heated gases. It is evidently due to heat generated 

 by them. 



Chemical change (methylosis) is more especially the change at or 

 near the surface of the earth, that is, from without. 



All rocks, no matter of what kind or of what age, are liable to be 

 affected by one or more of these kinds of metamorphic action, any one 

 of which may affect the rocks before or after the others ; but the order 

 in which they more commonly appear to occur is : first, contact ; 

 second, regional ; and afterwards, chemical change from without. 

 They may, however, occur so confusedly that, in places, the order is 

 more or less obscure. Therefore, to simplify matters, we shall at the 

 present time only consider the effects that regional metamorphic action 

 alone would produce. 



Any region or area of rock -masses may be subjected successively 

 to two, three, or more invasions of regional metamorphosis. For 

 example, a tract of Cambrian rocks may have been under such condi- 

 tions that they are metamorphosed ; wlaile subsequently these condi- 

 tions were removed, to be succeeded by others, the latter again 

 affording the conditions of further metamorphic action ; and this 

 may be repeated over and over again : so that the same rocks 



1 This has been already stated in chapters x. and xi. of my Geology of Ireland, 

 to ^^'hich I would call attention, as names used hereafter ; also details in regard to 

 metamorphic action, will be found therein, which it appears unnecessary to repeat 

 here. 



