18461856] CLEAVAGE AND FOLIATION 203 



account for the Royal Society. 1 I hope you will have a Letter 539 

 good illustration or map of the waving line of junction of the 

 slate and schist with uniformly directed cleavage and foliation. 

 It strikes me as crucial. I remember longing for an oppor- 

 tunity to observe this point. All that I say is that when slate 

 and the metamorphic schists occur in the same neighbourhood, 

 the cleavage and foliation are uniform : of this I have seen 

 many cases, but I have never observed slate overlying mica- 

 slate. I have, however, observed many cases of glossy 

 clay-slate included within mica-schist and gneiss. All your 

 other observations on the order, etc., seem very interesting. 

 From conversations with Lyell, etc., I recommend you to 

 describe in a little detail the nature of the metamorphic 

 schists ; especially whether there are quasi-substrata of 

 different varieties of mica-slate or gneiss, etc. ; and whether 

 you traced such quasi beds into the cleavage slate. I have 

 not the least doubt of such facts occurring, from what I have 

 seen (and described at M. Video) of portions of fine chloride 

 schists being entangled in the midst of a gneiss district. Have 

 you had any opportunity of tracing a bed of marble ? This, 

 I think, from reasons given at p. 166 of my 6\ America? 

 would be very interesting. A suspicion has sometimes 

 occurred to me (I remember more especially when tracing 

 the clay-slate at the Cape of Good Hope turning into true 

 gneiss) that possibly all the metamorphic schists necessarily 

 once existed as clay-slate, and that the foliation did not arise 

 or take its direction in the metamorphic schists, but resulted 

 simply from the pre-existing cleavage. The so-called beds in 



1 " On the Arrangement of the Foliation and Cleavage of the Rocks 

 of the North of Scotland." Phil. Trans. R. Soc., 1852, p. 445, with 

 Plates XXIII. and XXIV. 



2 " I have never had an opportunity of tracing, for any distance, along 

 the line both of strike and dip, the so-called beds in the metamorphic 

 schists, but I strongly suspect that they would not be found to extend, 

 with the same character, very far in the line either of their dip or strike. 

 Hence I am led to believe that most of the so-called beds are of the 

 nature of complex folia, and have not been separately deposited. Of 

 course, this view cannot be extended to thick masses included in the 

 metamorphic series, which are of totally different composition from the 

 adjoining schists, and which are far-extended, as is sometimes the case 

 with quartz and marble ; these must generally be of the nature of true 

 strata" (Geological Observations, p. 166). 



