112 PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. 



ously and marshalled so skilfully, is in favour of the idea that the 

 crystalline schists and gneisses were formed in Archsean ages under 

 conditions which have never subsequently recurred in any large 

 area of the globe. 



My own investigations, as wiJl have been inferred from the fore- 

 going remarks, have led me to consider a like conclusion as the most 

 probable. On the one hand the phenomena of contact-metamorphism, 

 on the other those of pressure-metamorphism, throw considerable 

 light on the genesis of crystalline schists. But they afford only a par- 

 tial explanation ; each illustrates rather than elucidates. Something 

 also may be learnt from the phenomena of veins — there, for example, 

 the formation of felspar, apparently so difficult in the other two 

 cases, appears to be easy. Still something more seems wanting, 

 some cause or causes, universal rather than local, general rather 

 than exceptional, the results of which in Palaeozoic and later periods 

 have been imitated rather than equalled. Heat, water, pressure, 

 have, no doubt, had their share in producing the result, but they 

 appear to have cooperated over vast areas of the earth's crust in a 

 way that seems inconceivable under existing conditions. 



Transitional Forms. 



It must be remembered that whichever hypothesis be adopted to 

 explain the orgin of the gneisses and crystalline schists, whether we 

 regard them as results of conditions prevalent only in Archaean 

 times, or as due to regional metamorphism acting on Pateozoic or 

 later deposits, transitional forms must exist. On the one hand 

 there would be a gradual approximation to the conditions which 

 prevailed at the commencement of the Palaeozoic series, which epoch, 

 it must be remembered, cannot be supposed to correspond with any 

 universal break in the history of the globe. On the other, we should 

 expect to find every graduation, from the most marked effects of 

 regional metamorphism till it became practically inoperative. Thus 

 some of the rocks in our own island which occur beneath the base 

 of the Cambrian, those properly called Pebidian, do not, to use the 

 ordinary phrase, exhibit much more metamorphism than those next 

 above in succession. But, as it happens, in those regions known to 

 myself, these transitional forms are rare, and it would seem as if, in 

 our own island, while the concluding page was torn away from the 

 Archaean volume, a whole chapter at least was missing from near 

 the end, of which, however, the careful searching of conglomerates 

 may restore to us some fragments. The break seems even greater in 

 those parts of Europe and Korth America of which I have any know- 

 ledge. I think, however, that this rarity of transitional forms — of 

 hypometamorphic rocks — is some argument in favour of the Archaean 

 age of gneisses and crystalline schists ; for if they were the result of 

 regional metamorphism acting on sediments of later date, transi- 

 tional forms should be common. Instances are numerous where the 

 Palaeozoic rocks have been depressed to great depths and buried 

 beneath thousands of feet of later deposits, or have been exposed to 

 the action of immense lateral pressure ; yet in any case where the 



