ANNIVEESAEY ADDKESS OF THE PEESIDENT. IO9 



district, in fact, would prove too much for the modern advocate of 

 regional metamorphism ; for he would have to explain the rapid 

 passage from a rather coarse gneiss to an ordinary black slate, besides 

 accounting for the fragments of gneiss in the intervening zone. 



I may also remark, after careful examination of a large number 

 of specimens, that the Alpine gneisses and schists, including even the 

 latest of those described above, seem to have assumed a thoroughly 

 crystalline condition before being subjected to the great pressures, of 

 which, in some cases, they now retain such conspicuous marks. In 

 regard to the coarser gneisses and older rocks, this obviously follows 

 from what I have said ; but, I believe, it is no less true of the 

 Tremola and later schists. The larger flakes of mica, the garnets, 

 the andalusites, cyanites, and other subsilicates which occur porphy- 

 ritically in finer- grained schists, possibly also the actinolite of the 

 Tremola schists (though here, as we might anticipate from a variety 

 of hornblende, there is something to suggest a recrystallization of 

 the constituents), appear to be anterior in date to the great earth- 

 movements. It is true that we cannot be certain that the latter 

 were pre-Tertiary, but as there is good reason to believe that all the 

 crystalline schists are pre-Carboniferous, the epoch of their crystal- 

 lization is thus thrown very far back, because the considerable size 

 of many of the above minerals, and the fact that inchisions of other 

 minerals are not extraordinarily numerous, shows that there must 

 have been much free movement among the constituent molecules of 

 the rock, and indicates that their aggregation must have taken place 

 under conditions very different from those of ordinary pressure- or 

 contact-metamorphism. 



The rocks also of the Ardennes, in the valley of the Meuse, are 

 often quoted as exhibiting instances of regional metamorphism. 

 These I know well, and have no hesitation in saying that the state- 

 ment is only true in the most limited sense of the words. It is 

 quite true that certain secondary minerals have been developed in 

 the green and black glossy slates, presumably of Cambrian age ; but 

 between these and the normal mica-schists, there is almost as wide a 

 gap as between a slate from near Torcross and a schist from the 

 Lizard. 



In Pembrokeshire the conglomerate at the base of the Cambrian 

 contains occasionally materials which, according to the ordinary rules 

 of geological reasoning, must have been derived from the so-called 

 Dimetian rock, as well as rare fragments of crystalline schists not 

 now exposed at the surface. The underlying tuffs (Pebidian) only 

 exhibit a development of mica on a very minute scale, and thus they 

 are no more than embryonic mica-schists. In Anglesey, conglome- 

 rates which cannot be newer than basement Ordovician, and may 

 be much older, contain indubitable fragments of the coarse granitoid 

 gneisses and some of the schists of that island. The coarser beds of 

 presumably Ordovician age in Cornwall afford indications of pre- 

 existent schists. The Charlton Hill conglomerate, near the Wrekin, 

 which is certainly much older than the Lingula Flags and probably 

 pre-Cambrian, contains Malvernian rocks and schists of ArchaBan 



