Prof. T. G. Bonney — Traverses of the Central Alps. 495 



tinuous deposition generally begins with some part of the Trias. 

 The conglomerates, which often occur at the base of the non-crystal- 

 line deposits, indicate that the principal metamorphism of tbe 

 crystalline series was anterior to both these epochs. There is at 

 present no reason to suppose that either in the Central Alps or for 

 some distance on each side there are any representatives of the 

 earlier Palceozoics. I believe that the conglomerates at the base of 

 the Carboniferous contain fragments of the later crystalline rocks 

 of the Alps as well as of some of the earlier — though I do not assert 

 that these crystalline rocks have undergone no modifications since 

 Carboniferous times. 



3. In the heart of the principal Alpine chains, and apparently 

 at the base of everything, are coarsely crystalline gneisses. These 

 diifer little from granites, except that they generally — almost alwa3's 

 — exhibit a certain foliation, and occasionally seem to be interbedded 

 with thin seams of micaceous schists or flaggy fine-grained beds. 



4. On examining these latter, we find reason to believe that they 

 are generally due to crushing. Their strike agrees with that of the 

 apparent foliation in these older rocks, and with that of a foliation 

 which is also present in the newer crystalline rocks. This corre- 

 sponds with the strike of the main physical features of the district, 

 and with the cleavage in the included troughs of sedimentary rock. 

 It runs for great distances with remarkable uniformity, e.g. from the 

 Maderanerthal to the upper part of the Lukmanier Pass the strike 

 of this foliation does not materially vary from W.S.W. — E.N.E. 



5. This apparent foliation is due to the development of extremely 

 thin films of a micaceous mineral. In many cases it causes the rock 

 to bear the aspect of a highly micaceous schist ; yet on examining 

 a transverse section, it is seen distinctly to be a crushed gneiss ; i.e. 

 though so conspicuous, it is a mere varnish. As it thus differs 

 materially from a true foliation, it would be convenient to give it a 

 name, and I should propose to call it the " sheen surface." It is, 

 in fact, a kind of "cleavage foliation," that is, a foliation due to 

 cleavage, and subsequent to it. But though from certain points of 

 view so conspicuous, its minerals often constitute a very small part 

 of the mass of the rock. 



6. The pressure which has produced this " sheen surface " has in 

 many cases aifected the orientation of the minerals, which are present 

 in the true " foliation " layers of the more distinctly foliated, i.e. 

 mineral-banded, rocks. It has affected these minerals as pressure 

 affects the constituents of a sedimentary rock. 



7. In the crystalline schists very commonly the " sheen surface " 

 corresponds with the original foliation surface, as in the slates the 

 cleavage sometimes does with the bedding. This is due to the fact 

 that the axes of the great folds often make a very high angle with 

 the horizon. It may, however (like a cleavage surface), be seen 

 crossing the foliation at all angles. 



8. Thus a non-foliated crystalline rock may be rendered to some 

 extent foliated by pressure (followed by a certain amount of mineral- 

 ization) ; i.e. some gneisses may be formed by crushing from granites, 



