ANNIVEESARY ADDEESS OF THE PRESIDEJ!fT. IO5 



downwards among the metamorphic rocks, the cause producing 

 molecular change has operated in the reverse direction, and that in 

 the gneisses and schists we have nothing more than ordinary sedi- 

 ments, which, in consequence of having been buried deep beneath 

 superincumbent masses of sediment, have been exposed under great 

 pressure in the presence of water to the action of a comparatively 

 high temperature. This ox)inion has found favour with many, and 

 is worthy of consideration ; it is in the highest degree probable that 

 some of the metamorphic rocks were once ordinary sediments; 

 moreover agencies such as those just named could not fail to 

 produce some effects, so that this may be the history — I would say, 

 probably is the history — of some of the metamorphic rocks. But 

 when to this proposition the corollary is added, '- Therefore the 

 metamorphic rocks may be strata which have been deposited in 

 Palaeozoic or even later times," I am obliged to assert that, while 

 admitting the abstract possibility, I have seen no direct evidence in its 

 favour. xUl attempts to identify large groups of metamorphic rocks 

 with strata of post-Archcean age have proved to be failures. Masses 

 of Palaeozoic rock have been buried deep, and exposed to the vertical 

 pressure of two or three miles of superincumbent rock, besides lateral 

 earth-thrusts, and the amount of mineral change produced, compared 

 with that in a true mica-schist, is trivial. The Skiddaw slate is said 

 to have been overlain by not less than twenty-five thousand feet of 

 rock*, no small part of which was formed hy volcanic agencies, and 

 yet it is only metamorphosed in the neighbourhood of the granites-. 

 The Old Eed Sandstones of South Wales have been buried beneath 

 at least ten thousand feet of Carboniferous rock ; but here, too, the 

 change is only mechanical. Yet, if metamorphic rocks had been 

 produced by the alteration in later geologic times of sediments of 

 known date, we should expect to find a gradual upward transition 

 from the metamorphosed to the unmetamorphosed rocks ; but this is 

 precisely what we do not commonly meet with, for below the latter 

 there is generallj^ an abrupt break. Rocks intermediate between 

 schists and shales are far more rare than we should expect in Nature, 

 supposing this hypothesis correct. 



Selective Metamorpliism. 

 By this is meant the variable effect of such agencies as pressure 

 and heat on rocks originally of different mineral constitution, so 

 that those composed of materials readily lending themselves to 

 change undergo great alterations, while the more refractory retain 

 their original characters. In the past, selective metamorphism has, 

 I think, been overestimated, and has been invoked by myself equally 

 with others too much as a deus e.cc machine/, to help us out of every 

 difficulty. At the same time there is a certain truth in it, which 

 must not be overlooked ; and forgetfulness of this, even on the part 

 of those who are in the habit of invoking it, has led sometimes to 

 erroneous inferences as to the history of a rock and to comparisons 

 between those which present essential differences. Thus in certain 

 * Ward, Mem. Geol. Survey, Lake District, p. 73. 



