140 



J. J. H. TEALL OX THE lIETAMOPvPHOSIS OF 



Witli regard to the development of foliation the evidence is very 

 satisfactory. The transition from, a crystalline granular rock to a 

 perfectly typical schist may occur in the space of an inch. The 

 specimen figured (fig. 1) was accidentally fractured, and a little of 



Pig. 1. — Block shoiving Passage from Granular to Foliated Structure. 

 (Three fourths natural size.) 



(Mechanisraus der Grebirgsbildung, Band ii. p. 55) calls attention to the fact 

 that in a cleaved ci'vstalline limestone belonging to the Hoch^ebirgskalk, the 

 individual calcite grains are of unequal dimensions in the different directions, 

 and have their longest diameters lying parallel to each other in the plane of 

 cleavage, although in the uncleaved rock the crystalline grains are of nearly- 

 equal dimensions in the different directions. He suggests that this is due to the 

 plastic deformation of the individual grains of calcite. Lehmann (Die 

 Entstehung der altkrystallinischen Schiefergesteine, p. 197) objects to this 

 view, and calls attention to the evidences of fracture and crush occurring in the 

 rocks investigated by him. The fact established by Heim requires an expla- 

 nation, however, and the most obvious one is the assumption that a molecular 

 rearrangement occurs in the calcite under the mechanical conditions which 

 determine the cleavage. The following important questions remain to be 

 solved : — 



Lender what conditions is the deformation of rocks accompanied b}' a crushing 

 of the individual constituents ? Under what conditions is it accompanied by 

 entire molecular rearrangement ? And lastly, "Under what conditions do these 

 two more or less opposite phases of metamorphism occur at one and the same 

 time? 



In the north-west of Scotland there are schists that have been formed by 



