Prof. T. G. Bonney — Foliation and Metamorphism. 199 



down, till a well-banded gneiss results from the movement of the 

 imperfect mixture (l). 1 (b) A similar gneiss occurs in Kynance 

 Cove and the cliffs to the south, though here the first stage is not 

 so well illustrated (2). (c) In Sark, on both the east and west 

 coasts, we find a similar banded gneiss, produced by the intrusion 

 of an aplitic granite into a basic diorite or hornblendite (3). The 

 hornblende schists of the Lizard and of Sark also suggest a similar 

 origin, viz., that after a moderately basic magma has been separated 

 by differentiation into a more acid and a more basic part, the two, 

 while still in a pasty state, have been forced to flow together (4). 

 The different stages (d) of this process are very well displayed on the 

 ice- worn rocks at the foot of the Allalin Glacier in the upper part 

 of the Saasthal (5). Here a rather fine-grained apiite breaks into 

 a mass of griiner Schiefer (probably a pressure-modified diabase). 

 Sometimes the one rock simply forms veins in the other; sometimes 

 the former breaks off fragments from the latter ; sometimes it melts 

 part of these, producing locally a fine-grained green-streaked gneiss. 

 A process generally similar may be studied (e) in the rocks under 

 Castle Cornet, Guernsey (6). Thus, as gneisses and hornblende- 

 schists, exactly resembling those of which the genesis can be 

 observed in the field and under the microscope, are by no means 

 rare, we seem to be justified in concluding that this production of 

 banded gneisses by partial mixture of more or less plastic magmas is 

 one of frequent occurrence. 



b. Foliation also may be produced in an ordinary holocrystalline 

 rock by the action of pressure, more or less definite in direction, and 

 sufficiently great to cause a partial crushing of the mass, which is 

 succeeded by the formation of new minerals. Sharply folded rocks, 

 such as often abound in mountain ranges, afford many examples of 

 this kind of foliation. Here the quartz is more or less crushed; so 

 also is the felspar, but this mineral, by the subsequent action of 

 water (doubtless when the pressure was diminishing), is replaced by 

 a mixture of granules of quartz and small scales of a white mica, the 

 latter generally lying at right angles to the direction of pressure. 

 Thus a granite may exhibit, all stages from an ordinary gneiss to 

 a mica-schist (7), and a porphyritic granite those from an augen- 

 gneiss to one characterized by thin and rather minutely crystalline 

 bands (8). The so-called protogine of the Alps, which, instead of 

 being the "first-born" of their crystalline rocks, is intrusive into 

 great masses of a more or less banded gneiss (sometimes rather rich 

 in biotite), affords excellent examples of this, as in the Mont Blanc 

 range (9), on the upper part of the St. Gotthard Pass (10), and in 

 more than one district of the Engadine (11). 



One form of "pressure-gneiss" deserves special notice, because, 

 till about thirty-five years ago, its origin was a geological puzzle. 

 Of this the so-called Newer Gneisses of the North- West Highlands 

 are an historic example. These appear to overlie the basal Cambrian 

 quartzite and some other sedimentary rocks belonging to the rest of 

 that period and perhaps a little of the Ordovician. They form an 



1 Figures in heavy type refer to the bibliography at the end of the paper. 



