208 GEOLOGY [Chap. IX 



Letter 542 in W. Tierra del Fuego, where clay-slate passes by alternation 

 into a grand district of mica-schist, and in the Chonos Islands 

 and La Plata, where glossy slates occur within the meta- 

 morphic schists, the foliation is parallel to the cleavage — i.e. 

 parallel in strike and dip ; but here comes, I am sorry and 

 ashamed to say, a great hiatus in my reasoning. I have 

 assumed that the cleavage in these neighbouring or inter- 

 calated beds was (as in more distant parts) distinct from 

 stratification. If you choose to say that here the cleavage 

 was or might be parallel to true bedding, I cannot gainsay 

 it, but can only appeal to apparent similarity to the great 

 areas of uniformity of strike and high angle — all certainly 

 unlike, as far as my experience goes, to true stratification. 

 I have long known how easily I overlook flaws in my own 

 reasoning, and this is a flagrant case. I have been amused 



Fig. 7. 



to find, for I had quite forgotten, how distinctly I give a 

 suspicion (top of page 155) to the idea, before Sharpe, of 

 cleavage (not foliation) being due to the laminae forming 

 parts of great curves. 1 I well remember the fine section at 

 the end of a region where the cleavage (certainly cleavage) 

 had been most uniform in strike and most variable in dip. 



I made with really great care (and in MS. in detail) 

 observations on a case which I believe is new, and bears 

 on your view of metamorphosis (p. 149, at bottom). 2 



In a clay-slate porphyry region, where certain thin sedi- 

 mentary layers of tuff had by self-attraction shortened 



1 " I suspect that the varying and opposite dips (of the cleavage- 

 planes) may possibly be accounted for by the cleavage-laminae . . . 

 being parts of large abrupt curves, with their summits cut off and worn 

 down " {Geological Observations on S. America, p. 155). 



" Ibid., p. 149. 



