Yol. 50.] CRYSTALLINE SCHISTS IN THE LEPONTINE ALPS. 301 



V. General Conclusions. 



After studying the sections which have been described above, I 

 spent some time in refreshing my memory of the ' Biindner schiefer,' 

 especially in the defiles of the Via Mala and the Schyn, and in care- 

 fully examining, by no means for the first time, cases where Meso- 

 zoic limestone, yet farther to the east, is infolded in crystalline rocks. 

 As the result, I repeat, if possible more emphatically than I have 

 ever done before, that the Mesozoic phyllites, such as those in the 

 Thusis district, 1 though bearing every indication of having been 

 subjected to severe pressure, are readily distinguishable from the 

 dark schists in the (upper) Crystalline group : the impure limestone 

 and hard sandstones interstratified with the former, from the calc- 

 mica-schists and the quartz-schists respectively, which form parts of 

 the latter ; and that the larger and purer masses of Mesozoic lime- 

 stone or dolomite, when they occur, notwithstanding their general 

 correspondence in chemical composition, are as different as they well 

 can be from the marbles and dolomites which are associated with 

 the above-named crystalline schists. The former, though often 

 brecciated, sometimes almost shattered by pressure, are compact- 

 looking, the latter saccharoidal. The only difficulty which arises in 

 distinguishing the two groups of rocks is one merely local, when 

 they happen to be exceptionally crushed in situ. The differences in 

 microscopic structure are not less marked than in the macroscopic 

 aspect. 



Since my visit to the Alps in 1889 I have made two fairly exten- 

 sive journeys in that chain (the former also in company with Mr. J. 

 Eccles), in course of which I have ' sampled ' the rocks bearing on 

 these questions in many places, from the Val des Ormonds on the 

 west to the Bernina Pass on the east, besides working at crystalline 

 schists and slaty rocks in other countries. Observations in the field 

 have been checked and tested by study with the microscope, using 

 it not so much for micro-mineralogical as for ' pathological ' purposes, 

 with the result that I am more than ever convinced of the general 

 accuracy of the conclusions which were expressed in my former 

 paper ' On the Crystalline Schists and their Belation to the Mesozoic 

 Bocks in the Lepontine Alps.' 



Discussion. 



Dr. J. W. Gregory remarked that the paper was a valuable con- 

 tribution to an important controversy, as it contained three new 



on their presence completely effaces the negative argument founded on the 

 absence of the third, in any discussion as to the relative age of the two groups. 

 Moreover. I have found fragments of black-garnet schist in a breccia at the 

 base of the admittedly Jurassic rocks of the Alp Vitgira (op. cit. p. 235). 

 Further examination enables me to speak yet more positively on this point than 

 I did in 1890. 



1 I accept these as Mesozoic on Prof. Heim's authority, but have not myself 

 attempted to fix their exact position. I am quite certain that they are much 

 later than such rocks as the crystalline schists of the Pennine Alps. 



Q. J. G. S. No. 199. y 



