64 Professor T. G. Bonney [April 4, 



but, notwitlistanding this, there is no very definite cleavage in the rock 

 mass, nor tendency to separate easily along the different mineral 

 layers. Specimens of such rocks may be obtained in the Alps, but there 

 are others in v^^hich the layers have evidently been crumpled up 

 after the period of mineral change : the bands of quartz and felspar 

 have been, as it were, crushed together, the flakes of mica are some- 

 times crumbled and sometimes twisted round into new positions. 



The subject is a technical one, so I must ask you to accept my 

 statement, without the long details of microscopic work on which it is 

 founded, that the older Alpine rocks frequently testify to having 

 undergone an extraordinary amount of crushing. In the middle of 

 coarse gneisses, for example, streaks and thin bands of a mica schist 

 may be found, which are not due to an original difference of materials, 

 but to the fact that here and there the original rock has yielded to 

 enormous pressure, and has been crushed in situ into lenticular bands 

 of rock dust, from which some new mineral developments have taken 

 place. You may notice also in some regions, where you would classify 

 the rocks at first sight as mica schists, that a close examination of the 

 broken surfaces at right angles to what appear to be planes of foliation 

 reveals a structure resembling a coarsish gneiss. The microscope 

 shows that the rock is really a gneiss, somewhat crushed, and that 

 the micaceous layers are of extreme tenuity — mere films, which do not 

 seem to have been original constituents. The gneissic mass has been 

 crushed, cleaved, and on the cleavage planes films of a hydro-mica 

 have been developed. We cannot fail to be struck, when once our 

 eyes have been opened to it, by the frequency of a slabby structure 

 in the more central parts of the Alpine ranges, the surfaces of these 

 slabs being coated with minute scales or films of mica. These i "^ 

 really records of a rude cleavage which has been impressed upon the 

 more central and less flexible portions of the Alps during the great 

 earth movements which they have undergone since they were first 

 metamorphosed. 



Thus in the building of the Alps our thoughts are carried very 

 far back in the earth's history, far beyond the earliest strata of the 

 Palaeozoic age. Under what conditions were these great homogeneous 

 granitoid masses of the fundamental gneisses formed ? They differ on 

 the one hand from granites, on the other from the ordinary gneisses ; 

 from the former their differences are but slight, and of uncertain value, 

 yet into the latter they appear to graduate. There is nothing like to 

 them in any subsequent rock group, and, so far as our knowledge at 

 present goes, they appear to be the records of a period unique in the 

 world's history. This may well be. When the dry land first ap- 

 peared, when the surface of the earth's crust had not long ceased to 

 glow, when the bulk of the ocean yet floated as a vapour in the heated 

 atmosphere, when many gases now combined were free, we can well 

 imagine that the earliest sediments would be deposited under con- 

 ditions which have never been reproduced. In the later schists, with 

 their more frequent mineral changes, their distinct stratification, and 



