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PKOE. T. Gr. BONNET ON THE 



the felspar often occurs in rounded grains up to the size of hemp-seed, 

 with a greenish-grey mica between. Still it is difficult to explain 

 the marked mineral banding of the rock, evidenced among other 

 things by the solidity of the paler, and the incoherency of the darker 

 bands, — exhibited both in their weathering and their behaviour when 

 hammered, the latter rock crumbling to pieces like an arkose, — unless 

 we assume that it existed prior to the crushing, and of this I think 

 we obtain, at least at one point, conclusive evidence. As a rule the 

 crush-planes and apparent stratification-planes are parallel, both 

 striking roughly east to west, with a dip of about 45° to the northern 

 side ; but at one place we have a section, of which a rough sketch is 

 annexed (fig. 1). Here we have the mineral banding perfectly well 



Pig. 1. — Diagrammatic sketch of Contorted Banded Gneiss at the 

 Embouchure of the Pouldu. (About 6 feet high.) 



The lines indicate the courses of the more conspicuous quartz-felspar bands ; the 

 interspaces the more micaceous parts, which haye a rude cleavage parallel 

 with the arrow. 



Pig. 1 A. — Small portion of one of the Contortions shown in Pig. 1, 

 about 8 inches in diameter, showing the quartz-felspar bands 

 with occasional cracks parallel with cleavage, the dotted part being 

 the more cleaved micaceous bands. 



defined, and forming a series of loops, the axes of which are paralle 

 with the usual direction of the banding and cleavage-planes, but at 

 the top and bottom of the loops the bands " wriggle " across the 

 direction of cleavage. Throughout the darker bands the cleavage 



