62' Professor T. G. Bonney [April 4, 



we justified in assigning to this zone, with those characteristics, a verti- 

 cal thickness of more than a mile ? To these questions I can give at 

 present no answer, further than to state that I am convinced that, not- 

 withstanding the apparent regularity of the bedding in this and 

 the neighbouring peaks, there are really great folds which patient 

 scrutiny may at length unravel, and that this zone of greenish rocks — 

 for which Alpine geologists have proposed the name of Pietra Verde 

 group, appears to underlie the garnetiferous series of silvery mica 

 schists, and either to overlie or replace the upper portions of the 

 banded gneiss series which succeeds to the fundamental series. 



I do not propose to weary you further with the details of Alpine 

 sections, except that I must add a few words upon the extent of this 

 remarkable series to which I have now introduced you. On the 

 northern side of the watershed in the Swiss Alps, so far as I am 

 aware, it is not generally strongly developed, except in certain 

 localities in the southernmost of the three ranges which make up the 

 whole chain, but in parts of the Tyrol it is well displayed. It 

 borders — the mica schists sometimes dominating — the fundamental 

 gneiss in the Oetzthal massif; it forms the peak of the Gross Glockner ; 

 it meets us on the Brenner Pass and elsewhere overlain by and 

 folded up with rocks which, if my memory do not mislead me, are the 

 equivalents of the Lustrous Schists of more western districts. 



Again, it is finely developed, seemingly in succession to bedded 

 coarser gneiss, in some of the peaks of the Bernina range, and it 

 occupies a considerable tract about the heads of the valleys to the 

 south. It may be traced, indeed, over a great zone, and with but slight 

 interruption all along the southern slopes of the Alps, even to the 

 south of the head waters of the Po, forming many of the grandest 

 peaks in the Graian, Tarentaise, Maurienne, and Cottian Alps ; and 

 we find traces of it overlying the coarse granitoid series in the massif 

 of the Alps of Dauphine. 



Sections, indeed, in the neighbourhood of Biella, according to 

 Gastaldi and Sterry Hunt, exhibit the Pietra Verde group overlying 

 the upper or more bedded portion of the great gneissic or basal series, 

 and succeeded by the group of friable gneisses, described above as 

 closely associated with the garnetiferous schists, in a manner that 

 suggests an unconformity. TJnder ordinary circumstances we should 

 not hesitate to admit that there is considerable evidence in favour of 

 this break, and some for one between the Pietra Verde group and the 

 stronger gneisses and schists below ; but in mountain regions we fear 

 to trust our eyes. The evidence, however, in certain districts in favour 

 of a break at the base of the Lustrous Schists is yet stronger. If I am 

 right in regarding the Lustrous Schists as forming one group with 

 the older part of the Bundnerschiefer of the Grisons region, and of 

 the Thonschiefcr of Von Hauer in the Eastern Alps, a study of the 

 geological map will show that it is difficult to explain the relation of 

 these beds to the underlying gneisses and schists without such an 

 hypothesis. What I have myself seen in regard to the Lustrous Schists 



