no PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. 



types. In the Highlands, notwithstanding the structural changes 

 which some of the Archaean rocks have undergone in subsequent 

 earth-movements, they can, as a rule, be distinguished from the 

 Palaeozoic strata, and I have no doubt that patient work by properly 

 trained observers will prove the same to be true of other districts. 

 As in the Alps, there will be occasional difficulties, small areas 

 where I^ature has obliterated all distinctive markings ; but these 

 prove no more than do gaps in an inscription, or obliterations of 

 any kind. If among a large "hoard" of shillings of the latter 

 part of the seventeenth century one were found worn perfectly 

 smooth, no antiquary would think this an argument for its being a 

 coin of George III. 



It is often asserted that fossils have been found in schists. In 

 some cases mere imitative markings have been mistaken for or- 

 ganisms ; in others, if the fossiliferous beds are not infolded between 

 older crystalline rocks, selective metamorphism has run wild, and 

 its advocates will find the old saying true, qui prouve trop, ne prouve 

 rien ; in others the rocks are not schists but slates. One case only 

 requires serious attention, the Norway micaceous schist with 

 Silurian fossils. Of this I have hitherto failed to obtain specimens 

 for examination, but from what I can learn, the rock is one of those 

 which is far nearer to the group of glossy slates than it is to the 

 true mica-schists. I may add that the theory of a regular upward 

 succession from the crystalline series to this fossiliferous rock is, on 

 Herr Eeusch's own showing, in the highest degree improbable. We 

 are, then, in this position, that the asserted equivalence of gneiss and 

 crystalline schists with Palaeozoic or later strata has in no single 

 instance been the subject of rigid proof ; that the evidence produced 

 in some cases rests only on a vague use of terms, in others has 

 broken down hopelessly on examination, so that while it would be 

 rash in our present state of knowledge to assert that this can never 

 be, we have a right to assume its improbability, to demand the 

 clearest proof, and a severe cross-examination of the witnesses. 



Origin of the Arclicean Rocks. 



If, then, as seems more probable, the great masses of gneisses and 

 crystalline schists are anterior to the Palaeozoic period, the question 

 naturally suggests itself. Under what circumstances were they pro- 

 duced ? If they differ materially from any rocks which have been 

 subsequently formed, it seems a fair inference that this is due to a 

 difference of environment ; if we perceive the nearest approach to 

 them in rocks which have been exposed, in the presence o£ water, 

 to exceptional heat or pressure, or to both, we should suppose that 

 they were formed at higher temperatures and under greater pressures, 

 in short under circumstances more favourable to chemical changes 

 and mineral development. On the hypothesis that this globe was 

 formerly an incandescent mass, such a state of things must have once 

 prevailed ; and as it cooled by the gradual radiation of heat into 

 space, there must have been an approach, probably at first somewhat 

 rapid, then more gradual, to the comparatively uniform condition 



