g6 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. 



an exceedingly fine-grained, foliated, micaceous quartzite ; until at 

 last, according to circumstances, our gneiss is converted into a very- 

 fine-grained, slightly foliated mixture of quartz, micaceous minerals, 

 and earthy or ferruginous materials, which might readily be taken for 

 an impure quartzite or quartz-schist, or else into one of the above 

 " sheeny " fine-grained micaceous schists. We have here, as may 

 be found in many other localities, an almost complete transitional 

 series ; but it is of course open to question, as I have already said, 

 whether the rock was originally a true granite or not, though in 

 any case its foliation cannot have been more marked than in some 

 of the Laurentian gneisses described above. There are, however, 

 in other districts, cases where the first stages of these changes, viz. 

 the production of a certain amount of foliation in a true granite, may 

 be observed, and these are similar to the earher stages described 

 above, though often not carried quite so far, so that it is needless to 

 dwell upon them in detail. Suffice it to say that the felspar crystals 

 lose their angular outline, and become more or less rounded, doubt- 

 less by a crushing away of the angles. The original micaceous 

 constituents appear to undergo some amount of rearrangement at 

 right angles to the pressure, a rude wavy cleavage becomes per- 

 ceptible in the same direction, and becomes coated by the usual 

 filmy white mica, which also, as before, is developed throughout the 

 rock. Hence I have no doubt that many of the augen- gneisses, 

 where the " eyes " are rounded crystals of felspar, are only porphy- 

 ritic granites modified by subsequent compression. 



When a rock which originally possesses a well-marked banding or 

 foliation, is exposed to pressure, much more varied results will be 

 produced. I have already described * how in the case of a fine- 

 grained banded sedimentary rock the mineral banding appears to 

 be intensified by a pressure acting at right angles to it, while it may 

 be bent and ultimately undergo strain-slip cleavage by one acting at 

 a high angle, or else (I think, if the pressure acts at a smaller 

 angle) may be in places obliterated and converted into a kind of 

 lenticular structure, recalling certain gneisses, in which, though 

 there is a foliated structure, there is no marked mineral banding. 

 Gneisses and schists undergo somewhat similar modifications, which 

 can be fully studied in the Alps, in Scotland, and in many other 

 mountain-regions, past and present. In the first-named two I have 

 given reasons for considering that the primary foliation of the rocks, 

 where distinguishable, corresponds with bedding. If we examine 

 some of the Alpine schists or gneisses where mineral banding is 

 conspicuous, we shall find that the pressure due to the earth-move- 

 ments has acted upon these in different directions, and with varying 

 intensity. Sometimes there will be little sign of disturbance, some- 

 times the pressure has been perpendicular to the stratification- 

 foliation, and gives to the layers a flattened or rolled-out lookf. 

 Sometimes these are bent and zigzagged, and folded in the most com- 



* " On the Geology of the South Devon Coast," Q. J. G. S. vol. xl. p. 18. 

 t Indeed they sometimes suggest the action of lateral tension rather than 

 of vertical pressure, an action possible, locally, in a folded region. 



