IV.—Some Notes on Gneiss

Extract

Much light has been thrown of late years on the history of gneiss and other foliated rocks. Geological literature is beginning to bristle with such terms as regional–metamorphism, pressure–metamorphism, dynamo–metamorphism, and the like. Yet longer and less intelligible names will probably follow, which, under the guise of precision, will foster confusion. So it may be a little help to students to set down, as far as possible in plain English, the fruits of some years' study of gneisses and allied rocks.inthis work I have taken nothing on trust, and have tested every inference to the best of my ability. Probably there is nothing original in the results, but they are all the outcome of personal observation, for I have always preferred questioning Nature to reading books. So,inorder to economize time in searching for what has been already said, and to save studding the page with references, I will assure the reader that he is quite at liberty to suppose that “everything has been said by somebody, somewhere.”

Let us begin our observations with a rock which is a fairly coarse granite, occurring in a region affected by great earth-movements, such as mountain-making, which, however, for some reason or wither, has escaped with its structure practically unmodified. Such a rock sometimes may be locally indistinguishable from an ordinary granite, but very commonly a small difference is perceptible, especially on a slightly weathered surface. This has a rather fragmental aspect, the quartz and the felspar presenting a superficial resemblance to unrolled clastic grains,—asinan arkose, the materials of which have been transported only for a short distance. This resemblance does not disappear on microscopic examination. The rock,inshort, has been fractured but not crushed. This structure is exhibited (not to mention other examples) locally in the granite at the entrance of the Yal Rosegg (Pontresina), in the granitoid rock of Twt Hill(Carnarvon), and of Llanfaelog (Anglesey); occasionally also in the so-called Dimetian of St. Davids. This, then, may be regarded as the first stage in pressure-modification.