120 FIELD GEOLOGY. 



often be seen somewhat smoothed and finely striated in 

 a direction generally almost vertical to the bedding; 

 this is the result of one portion of the rock sliding down 

 over the other during the progress of the fracture. But 

 a similar appearance is sometimes observable which 

 evidently is not due to friction, as the striae (or rather 

 fibres) are continued into the substance of the rock ; this 

 may be due to " crystallisation in parallel fibres," or to 

 recrystallisation as pseudomorphs after fibrous aragonite. 

 The reality may easily be determined by removing a 

 portion of the striated surface ; if the striated structure 

 be apparent within, the striae are due to crystallisation 

 if not, to friction ; and the fact should then be noted 

 as evidence of some movement of disturbance. 



Note on Metamorpliic Rocks. It is generally a diffi- 

 cult matter to decide whether or not a rock owes its 

 present character to the agencies of metamorphism. 

 Hocks originally deposited as sedimentary strata, and 

 having afterwards been altered, behave on a large scale 

 as do the unaltered deposits ; , e.g. " they never, like 

 (eruptive) Granite or Trap, send veins into contiguous 

 formations. In Great Britain those members of the 

 series which approach most nearly to Granite in their 

 composition, as Gneiss, Mica-schist, and Hornblende- 

 schist, are confined to the country north of the rivers 

 Forth and Clyde" (Lyell). 



Eounded water-worn grains will be found in most 

 metamorphic rocks, and distinguish them from igneous, 

 when not too compact for the grains to be visible under 

 a lens ; and whatever they may at one time have en- 

 closed, they will now be found " wholly devoid of or- 

 ganic remains." 



