1846— 1856] CLEAVAGE AND FOLIATK 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 cleava iation. 



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 arc uniform : of this I have seen 

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

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

 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 ha v 

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

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

 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 S. ./ 

 would be very interesting. A suspicion has sometimes 

 occurred to me (I remember more especially when tracing 

 the clay-slate at the Cape of Go id 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. 



' " I have never had an opportunity of tracing, for any distance, aloi 

 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 ti the 



nature of complex folia, and have not been separately dep sited. < 't 

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

 metamorphic series, which are of totally different composition from the 

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

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

 strata" {G. Observations^ p. 166). 



