ANJ^^IYERSART ADDRESS OF THE PRESIDEjS^T. IO7 



Moreover, when this rock has been exposed to great pressure, the 

 result appears to differ from that afforded by a gneiss under similar 

 circumstances. The quartzites also, when similarly treated, still 

 retain a certain individuality and differ from the quartz-schists of 

 presumed Archaean age. Apart, then, from the fact that the " newer 

 gneiss series " of the North-west Highlands retains its distinctive 

 features, and presents us with similar gneissoid rocks, not only in 

 regions where Torridon Sandstone is largely developed, but also in 

 those from which it is practically absent, I am of opinion that even 

 if newer materials are locally infolded with strata of earlier date, it 

 will, as a rule, be possible, on careful examination, to distinguish the 

 crushed-out Archaean from the crushed-out Palaeozoic members. 



Age of the Alpine Cleavage-foliation. 

 It may be asked What is the date of the secondary or cleavage- 

 foliation in the Alps ? In the district of which I have spoken, it 

 preserves a uniform direction over considerable areas, and its strike 

 appears to coincide generally with the strike of the cleavage in the 

 Secondary rocks, and that of the great folds by which the whole 

 region is affected. Hence it would seem to be post-Secondary and 

 probably post-Eocene. This, however, may not be universally true ; 

 for I know of cases which lead me to suspect that a cleavage- 

 foliation existed prior to the great period of subsidence, and the 

 upraising of the Alps was a more complicated process than is some- 

 times supposed. On this question, however, I do not at present 

 feel qualified to enter. I pass it by, merely remarking that the 

 development of minerals along the crush-planes only takes place 

 on an important scale in rocks originally crystalline, and even there, 

 although the general effect is plain, the individual mineral consti- 

 tuents by which it is produced are for the most part extremely 

 minute. It is a remarkable fact that, in a certain sense, a sheen 

 surface is often more conspicuous to the naked eye than it is under 

 the microscope. 



Age of the Gneisses and Crystalline Schists. 

 A few years since, the following sentence (which is quoted from a 

 very recent text-book) would have been accepted by most geologists 

 without hesitation : — " There are wide regions in which crystalline 

 schists (a) overlie fossiliferous strata*, or (6) contain intercalated 

 bands in which fossils occur, or (c) pass either laterally or vertically 

 into undoubted sedimentary strata." This conclusion is "confidently 

 drawn " from a series of statements of which, to my personal know- 

 ledge, several are uncertain, others are either incorrect or inadequate 

 for the purpose. I must again repeat that observations on such 

 questions as the relations of the crystalline schists and stratified 

 rocks, as to apparent similarities or dissimilarities, are of little value 

 unless they have been made by obseiTcrs well practised not only in 

 the field but also in the use of the microscope ; hence the statements 

 of the best observers, in what we may call premicroscopic days, 



* The author means in chronological sequence, not by inversion or overthrust. 



