ATOIVEESART ADDRESS OF THE PRESIDENT. 95 



similar instances exist in Scotland, among the slaty rocks to the 

 south of the zone of the crystalhne schists of the Central Highlands. 



Subsequent work on the part of several investigators has 

 thrown additional light upon the second kind of cleavage mentioned 

 by Dr. Sorby in his Address, that in which the cleavage-planes cut 

 across the undulating bands of the constituent minerals. Of this 

 structure I possess one or two excellent examples, e. g. from near 

 Torcross, which makes it clear that the structure is an example of 

 the strain-slip cleavage (Ausweichungs-Clivage) of Dr. Heim *. 

 These planes of weakness, both in this and in ordinary cleavage, as 

 noticed by Dr. Sorby, are marked out by secondary mineral deposits. 

 It is needless to enter into a minute description of the effects of 

 pressure upon less homogeneous fragmental rocks, since, so far as 

 they bear on my present subject, I shall have to refer to them in 

 speaking of the crystalline rocks. 



The effects of pressure upon these last are onl}- beginning to be 

 fully realized. It has long been known that pressure or strain, 

 definite in direction, acting during the solidification of the rock, 

 modifies the position of its constituents ; but it is only lately that 

 the effects of these on rocks already crystalline have been appre- 

 liended. Let us take as an example the effect of pressure on a holo- 

 crystalline rock such as the granitoid gneiss already mentioned as 

 forming the central part of the Bernese Oberland. Excellent ex- 

 amples may be found in the gorge of the Eeuss or near the Furka 

 road, in the neighbourhood of the Rhone glacier. We can there 

 trace a gradual sequence from the coarse granitoid gneiss to very 

 fine-grained slabby bands, the surfaces of which are coated with 

 extremely minute scales of silvery mica. The original constituents 

 of the rock are mainly quartz, more than one kind of felspar, and 

 biotite. If we examine under the microscope a specimen taken 

 from a part where the effect of crushing is beginning to be con- 

 spicuous, we find that the felspars are cracked or broken across ; the 

 fragments are displaced from their original position, they seem to 

 tail off into a mass of microliths of a white mica, evidently formed 

 from the finer detritus, bordering the larger grains, crowding cracks 

 in them, sometimes interspersed throughout their whole mass, until 

 it has almost lost its distinctive characteristics. The larger grains of 

 quartz are also cracked and broken, and with polarized light exhibit 

 the usual strain-phenomena. Much secondary quartz of chalce- 

 donic aspect has formed in little streaks and elongated pockets. 

 The biotite seems to have been pushed about, has often a "tattered" 

 look, and sometimes seems to have recrystallized ; a wavy, len- 

 ticular cleavage-structure at right angles to the direction of pressure 

 has become conspicuous, showing sheen-surfaces to the naked eye. 

 As we examine specimens further away from the uiicrushed rocks, 

 we find the felspar gradually disappear, all the constituents of the 

 rock become more and more minute, and thus it assumes a more 

 homogeneous character ; everything seems to have broken up, and it 

 bears some resemblance to one of the phyllitcs described above, or to 

 * Mechanismus der Gebirgsbildung, Taf. xv. fig. 11. 



