Dr. Du Riche Preller — Three Glaciations of Siviizerland. 27 



Pegmatite bands develop in increasing proportion as we approach 

 tlie zones of maximum metamorphisra, so that, where their develop- 

 ment has proceeded to any great length, they impart to that 

 particular zone of rock tbe appearance of bands of crystalline 

 felspar, etc., graduating at their sides into other bands, which 

 consist of more or less distinctly granulitized schist. A closely- 

 welded rock compound of that nature is, in all essential respects, 

 a truly foliated rock, and it is difficult to state in what respect it 

 differs from many rocks that are comprehended under the term gneiss. 



In other words, granulitic structure develops into augen ; augen 

 extend until they pass into pegmatite ; pegmatites aggregate until 

 they impart to the compound, of which they form a part, the 

 character of a true gneiss. 



Augen structure in rocks is, according to this view, one of the 

 earlier modifications whose later developments lead, in the one 

 direction, into the rocks of eruptive origin, and, in the other 

 direction, into truly foliated rocks, such as gneiss. The difference 

 in the final result in either case is due to the mode in which the 

 relief of pressure acted upon the rocks while they were in poten- 

 tially molten condition, arising from dynamic causes. The pressure 

 was relieved abruptly, and entirely, in the case of the eruptive rocks, 

 and by very slow degrees and, only partially, in the case of the 

 foliated rocks. 



This view of the origin of certain gneisses by dynamo-fusion does 

 not preclude the adoption of the theory of the formation of other 

 gneisses by differential movements acting upon an igneous mass in its 

 later stages of original consolidation. This is so evidently the mode 

 of origin of many undeformed gneisses that I have for some time past 

 employed for different types of original gneiss such names as granite 

 gneiss, granitite gneiss, syenite gneiss, diorite gneiss, gabbro gneiss, 

 etc., to express this fact. Nor does it preclude the possibility of 

 some forms of gneiss having originated in any other manner 

 not here referred to. Nevertheless, I venture to submit that 

 this view of the common origin of certain eruptive rocks and 

 gneisses by means of variations in the relief of pressure upon 

 superheated masses may fairly claim to rank as a useful working 

 hypothesis, seeing that it enables us to account for the intimate 

 association, on the one hand, of gneiss with true schists, and on the 

 other with the eruptive rocks, both plutonic and volcanic ; and also 

 that it presents a satisfactory explanation of the intimate association 

 of gneisses with the deeper-seated cores of mountain ranges, of 

 whatever age, or wheresoever situated. 



V. — On the Three Glaciations in Switzerland. 



By C. S. Du B.ICHE Preller, M.A., Ph.D., A M.I.C.E., M.I.E.E., F.C.S.,F.G.S. 



UNTIL very recently it was thoroughly recognised that in post- 

 Tertiary, viz.. Pleistocene or Diluvial times, there occurred 

 in Switzerland, as elsewhere, two glaciations which covered the 

 Alps and the greater part of the Swiss lowlands, and between 



