90 



PROCEEDINGS OE THE GEOLOGICAL SOCIETY. 



many laminar crystals of pale green mica, crumpled up and con- 

 torted by lateral pressure, like the thin beds in the slate of Lis- 

 keard, shown in fig. 4. As in that slate, many planes of discon- 

 tinuity have been formed by fracture ; and along these larger and 

 more colourless crystals of mica have grown, with their faces and 

 cleavage parallel to the walls, just as in some of the slightly altered 

 slates previously described. Passing to another specimen, the 

 mica, thus formed along the planes of cleavage-discontinuity, in- 

 creases in amount until, in some parts, it altogether preponderates 

 over the contorted laminae which are roughly parallel to the bedding, 

 and we see a foliation due to large plates of mica lying in a plane 

 corresponding to true slaty cleavage, inclined at a high angle to the 

 stratification. In other specimens the large irregular crystals of 

 both light- and dark-coloured mica lie almost wholly in the plane 

 of cleavage, but yet here and there are a few which have obeyed 

 the influence of the stratification. In a splendid specimen from 

 Portlethon the cleavage-foliation is very perfectly developed, and 

 is not only parallel to the axis planes of the larger contortions, but 

 has all the other characteristic relations of true slaty cleavage. As 

 an illustration of this kind of foliation I subjoin fig. 11, which re- 

 presents (magnified 30 linear) a part of a thin band of mica-schist, 

 where the foliation and bedding are at right angles to one another. 

 As in the stratification-foliation, shown by fig. 10, the study of the 

 detail shows very clearly that dark- and light-coloured mica and 

 quartz have all crystallized in situ ; but it is unnecessary to base 

 the argument on the minute detailed arrangement of the crystals, 

 since no one would suppose that plates of mica could be mechani- 

 cally deposited sticking upright, and still remain almost parallel 

 after the rock had been contorted. 



Fig. 11. — Mica-schist of Portlethon^ with cleavage-foliation. 



I hough I have never seen this kind of foliation better developed 

 than in the locaHties named above, I have also met with it in some 



