﻿224 DE. H. C. SOEBY ON THE APPLICATION OF [May I908, 



facts. In a considerable number of different localities I have met 

 with fine-grained, spotted slate-rocks; and, on careful examination 

 with the microscope, it was easy to see that the spots are small 

 imperfectly-developed crystals of chiastolite, somewhat broken up 

 and compressed, with their long axes in the plane of cleavage. 

 The relation between these and the surrounding rock clearly shows 

 that, when slaty cleavage was developed, the rock was nearly as 

 hard as chiastolite, yielding little more to compression. 



As already shown, the best slates of Penrhyn prove that, in the 

 line of dip, portions of rock which before cleavage were 100 : 100 

 are now on an average 100 : 705. The small flakes of mica, or a 

 mineral of similar character, which constitute so large a portion of 

 the rock, may be assumed not to have changed their dimensions, 

 and thus the entire yielding of the rock must have been due to the 

 giving-way of the surrounding finer particles. There must conse- 

 quently have been a great relative movement ; and, assuming as a 

 fair average that the flakes of mica form one half of the rock, the 

 surrounding material must have been elongated along the line of 

 dip from 1 to 1 to 1 to 14, whereas the flakes were not elongated 

 at all. It is thus easy to understand why there is so vast a 

 number of minute slip-surfaces perpendicular to the pressure 

 among the ultimate constituent grains in the line of dip, and why 

 such surfaces are much less developed in the line of strike, where 

 little elongation could occur. A number of these facts were over- 

 looked in my early papers, and they show that the compression of 

 the rock would alter its structure much more than I suspected. 

 The change in the position of the flakes would be twice as great as 

 I calculated. Instead of half of them being as before cleavage 

 spread over 90°, they would after compression be spread over only 

 8° ; and this, combined with the numerous small slip-surfaces, 

 easily explains the fissility of the rock. 



Slate near Penrhyn, which has been much changed in appearance 

 by a trap-dyke, still shows well-marked traces of the previously- 

 existing slip -surfaces, so that the rock cannot have been softened 

 by heat. In the mica-schist between Aberdeen and Stonehaven, 

 with cleavage-foliation and therefore metamorphosed after com- 

 pression, all traces of slip-surfaces have been obliterated. 



XYIII. Sfkfaces of Pkessuee-Solijtion. 



In my Bakerian lecture to the lioyal Society ^ I showed that 

 pressure increases the solubility of the greater number of soluble 

 salts. Also in my paper on the impressed limestone-pebbles of the 

 Nagelfluh " in Switzerland, I showed that the impressions could 

 not have been produced by simple mechanical action, since it is 



1 Proc. Eoy. Soc. vol. xii (1863) p. 538. 



2 Neues Jahrb. 1863, p. 801. 



