314 Trof. Bonney — Some Notes on Gneins. 



fund of bases, and not at the expense of the chlorite. Moreover, the 

 fused rock, unless it were subjected to shearing, or fluxion, in a 

 partially crystallized condition, would have consolidated as a granite, 

 not as a gneiss ; and would have exhibited a granitic and not a 

 foliated or gneissic structure. 



[The Editor regrets that owing to the pressure on the space of the Magazine, the 

 publication of this article has been unavoidably delayed. — Edit. Geol. Mag.] 



IV. — Some Notes on Gneiss.^ 



By Professor T. G. Bonney, D.Sc, LL.D., F.R.S. 



"UCH 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-metarnorphism, 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. 

 In this 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, in order 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 

 other, 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 super- 

 ficial resemblance to unrolled clastic grains, — as in an arkose, the 

 materials of which have been transported only for a short distance. 

 This resemblance does not disappear on microscopic examination. 

 The rock, in short, has been fractured but not crushed. This 

 structure is exhibited (not to mention other examples) locally in 

 the granite at the entrance of the Val Eosegg (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. 



Next suppose this agent to have produced more marked effects, 

 and the rock to have been definitely crushed, perhaps also slightly 



1 It may be well to state that this pnper was written some time prior to the 

 publication of Mr. Goodchild's paper in the January Number of this Magazine, and 

 has been left unaltered. 



