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). 



2 Ibid.) p. 149. 



