Prof. T. G. Bonney — Foliation and Metamorphism. 201 



probandi lies on anyone who claims a sedimentary origin for a gneiss, 

 whether banded or not, and in the few cases where this can be 

 proved the gneiss is usually limited in thickness and not quite 

 normal in character; sometimes, indeed, the supposed gneiss is found 

 on closer study to be merely a rolled out sill or vein of intrusive 

 granite (19). Thus it is prudent, in dealing with gneisses which 

 show no signs of passing, on the one hand, into schists by a gradual 

 transition, on the other into indubitable granites, to carry them for 

 the present to a " suspense account". 



I pass on now to those crystalline schists which can be proved to 

 be metamorphosed sedimentary rocks, viz. the group of the quartz- 

 schists, quartz-mica-schists, calc-mica-schists, and crystalline marbles 

 (or dolomites). These sometimes overlie, with marked unconformity, 

 the above-named gueisses, though occasionally it is difficult to divide 

 them. The same may be said of their relation to the overlying 

 bypometamorphic or unmetamorphie strata, but in many cases their 

 division from the one or other is not quite so clear. 



The sedimentary origin of these crystalline schists, if we except 

 the griiner Schiefer (and we must not exclude the possibility of some 

 of these being metamorphosed tuffs), is unquestionable. In chemical 

 composition they agree with corresponding members of the stratified 

 rocks, and are associated in exactly the same way. Of these associa- 

 tions striking instances may be found in the Val Canaria, Val Piora, 

 on the Nufenen and the Gries Passes, in the neighbourhoods of Binn, 

 Saas, Zermatt, and .at sundry localities in the French, Italian, and 

 Austrian Alps (20). Here we can find dark or lead-coloured mica- 

 schists, sometimes containing garnets (which maybe as large as peas) 

 interbanded with quartz-schists or passing rapidly into calc-mica- 

 schists and marbles. They show the effects, often conspicuously, of 

 subsequent pressure (21), but careful study makes it obvious that 

 before this acted they had been well metamorphosed, that change 

 having been due to the joint action of pressure, water, and heat, of 

 which probably the last agent was most above the normal. "We 

 infer this from the fact that stratified rocks, which must have been 

 depressed to very considerable depths from the earth's surface, and 

 have been kept for a long time sodden with water, do not exhibit 

 this kind of metamorphism. Its occurrence has often been con- 

 fidently asserted, but every instance, such as the " f ossiliferous 

 schists" of more than one part of the Alps and the "Devonian 

 schists" of the Start district, has broken down, on careful study in 

 the field and with the microscope, like the " Newer Gneisses " of the 

 Highlands. 



A quartz-schist consists mainly of grains of quartz, but usually 

 contains small flakes of white or lead-coloured mica, which 

 occasionally form distinct and rather conspicuous bands (22). 

 The purer varieties are almost white and slightly slabby (perhaps 

 a superimposed structure. When this schist rests on gneiss its lower 

 part occasionally contains some fragments of felspar and (in a few 

 cases) pebbles of vein quartz or some compact rock resembling 

 a halleflinta (23). Evidently the purer varieties much resemble 

 quartzites, but they present under the microscope a different 



