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of such events produced the laminated character of the 

 interstratified veins. It is also probable that in many cases 

 the quartz was deposited in the slate along a number of 

 parallel planes lying close together in an area of minimum 

 pressure and that the quartz films increased in thickness 

 through a widening of the spaces either by the folding of 

 the strata or by metasomatic replacement. 



The origin of the corrugations is more difficult of explana- 

 tion. That they are dependent on the rock folding is 

 generally conceded and the following explanation has been 

 adduced : — Many veins were formed long before the folding 

 process was completed and during the subsequent stages 

 they were subjected to the same forces as the rocks. The 

 main forces that produced the folding were horizontal, and 

 if the horizontal forces be resolved into components perpen- 

 dicular to and tangent to the bedding plane, the component 

 perpendicular to the bed will be greater on the limb than 

 at or near the apex of the anticline. There will thus be a 

 tendency towards a thinning of the beds on the limbs and 

 a proportionate thickening at and near the apex. This will 

 express itself in a motion of the more plastic beds, that is of 

 the shales or slates, from the limbs towards the apex result- 

 ing in a thickening on the latter, a phenomenon that is 

 frequently observed especially in closely folded strata. 

 Any quartz vein already formed in such slate will partake 

 of the same lateral motion; on the limbs of the fold where 

 the strata are not curved they will suffer little change, but 

 where the strata begin to curve and the slate beds to 

 thicken, the veins will begin to fold and produce corruga- 

 tions. On the side of long domes there was but one 

 deforming force that came prominently into play, that 

 which produced the east and west folding, and the corruga- 

 tions are consequently horizontal and parallel to the axis; 

 but at the pitch of the dome a second force about at right 

 angles to the first expressed itself in the pitching of the 

 dome, the resulting movement was more complex, and 

 corrugation was produced radiating more or less from the 

 centre of the dome. 



This theory of the origin of corrugations would also 

 account for the different degree of crenulation often 

 exhibited by different veins intercalated between the beds 

 composing one single slate belt; the more corrugated veins 

 being generally older than less corrugated ones. The 

 quartz appears to have become plastic under the enormous 



