G. H. Kinahan — Archcean Hocks. 503 



tic-looking mass composed of rounded pieces of rock differing in 

 lithological character both from the matrix and from one another, 

 occurring along what looks like the strike of the rocks, and yet may 

 be able to make out that it is entirely a superinduced structure due 

 to brecciation in place and subsequent decomposition of the broken 

 rock. 



From these observations we may also learn another lesson, viz. 

 that crushing may take place without producing any effects analogous 

 to metamorphism properly defined, and may act in aid of true meta- 

 porphism or other chemical change, chiefly by comminuting and 

 preparing the rock for chemical reaction. 



V. — Arch^an Rocks. 



By G. H. Kinahan, M.R.I.A., etc. 



(Read before the Eritish Association, Manchester, 1887.) 



AT the present time the American geologists, both those of the 

 United States and the Dominion of Canada, are at variance 

 as to the groups the Archeean should be divided into; it is however 

 unnecessary to enter into this controversy, as the strata may be con- 

 veniently grouped, as in Selwyn's map (1884), into Norians or 

 Lnhradonans, excessively coarse gneiss or other granitic rocks ; 

 Laurentians, gneiss and granite ; and Huronians, schists with sub- 

 ordinate gneiss. 



Some of the authorities both in the States and the Dominion 

 ■would have us believe that the Ai'chgeans originally accumulated as 

 crystalline foliated rocks ; that is, the minerals were deposited by 

 chemical action from solution. Others, however, hold that at one 

 time they were ordinary sedimentary rocks, which during one or 

 more successive periods of metamorphic action were changed into 

 their present conditions. The present school of English Archaean 

 geologists seem fi'om their writings to coincide with the Americans, 

 who believe in originall}^ crj'stalline accumulations. 



The facts in favour of the first theory seem to be obscure ; we 

 are indeed told that "chemistry forbids" an ordinary sedimentary 

 rock changing into a schist, gneiss or granitic rock ; yet in the field 

 we can walk along beds that change from unaltered rocks through 

 schist into gneiss, and thence into metamorphic granite. Such a 

 change may be quite contrary to the chemical laws laid down by 

 such Archgean geologists for their own guidance, but they are patent 

 facts effected by the Great Chemist, and to be ocularly learnt by 

 any ordinary observer who carefully works out any large tract of 

 metamorphic rocks. 



In connection with this subject the value of lithological charac- 

 teristics may first be mentioned. It is now generally well known 

 that any rocks, no matter to what period they belong, from the 

 newest Tertiaries to the Archaean, if they have been subjected 

 to sufficient metamorphic action, will be changed into schist and 

 gneiss ; that is, that foliation will be developed in them, although 

 originally they may have been ordinary sedimentary rocks, volcanic 



