VI.—On a Supposed Case of Metamorphism in an Alpine Rock of Carboniferous Age

Extract

A few years since it would have been flat heresy to assert that very clear proof would be necessary before we could accept a crystalline schist as the metamorphosed representative of a rock of Palæozoic age. Yet at the present time many who have made a special study of this branch of petrology would not hesitate to go thus far, and some would even declare that we do not know of any completely metamorphic rock which is not of Archæan age. Certainly the stock instances of metamorphism in Wales, and especially in Anglesey, in Cornwall, in Leicestershire, in Worcestershire, have utterly broken down on careful study. Outside the English Geological Survey probably no person who can use a microscope believes that the schists of Anglesey are altered Cambrian, or that the slates of this age are melted down into the quartz-porphyry of Llyn Padarn. It is becoming evident that even the metamorphic fastnesses of the Highlands are in danger, and that at any rate even there the realm of “altered Lower Silurian” will be grievously curtailed. Startling facts are now and then adduced by the defenders of what we may call the ‘established’ (i.e. non-progressive) geology; fossils are said to have occurred in crystalline non-calcareous rocks, Calamites in gneiss, Trilobites in mica-schist, and so on; but those who are familiar with the molecular changes which take place in the formation of such rocks as these will require the clearest evidence before they can accept statements so antecedently improbable.