84 PEOCEEDTNaS OP THE GEOLOGICAL SOCIETY. 



Fine-grained Schists. 



It will thus be seen that in the rocks just described we have a 

 gradual and perfect passage from, an almost normal slate to what 

 is practically a true mica-schist, with stratification-foliation or 

 cleavage-foliation. The only essential difference between the more 

 altered specimens and the most typical mica-schists is that the 

 crystals are less, being, on an average, only -^ as large. There 

 are, however, many beds amongst the Highland schists which are 

 of very little coarser grain than these altered slates. As far as I 

 can see, we have a real and perfect passage from slate to schist ; 

 and the more altered varieties from Wicca Pool do not differ from 

 the most typical schists in any more essential particulars than 

 pebbles differ from boulders. 



Eocks which are thus practically fine-grained schists differ so 

 little from true unaltered slate in general appearance, that probably 

 they have often been mistaken for slates. At all events, I know 

 that I must have many times made this mistake myself, which has 

 prevented me from being able to say any thing very definite re- 

 specting the horizontal distribution of the altered and unaltered 

 rocks. T regret that I cannot decide whether it is in all cases 

 merely a local change, or sometimes very general over a consider- 

 able district. 



Foliation in slightly altered Slates. 



On the whole, then, these connecting-links between slates and 

 schists clearly show that foliation is essentially different from either 

 stratification or cleavage. In my opinion they prove that it is the 

 result of the varying segregation of different minerals, controlled 

 by various previously existing structures. It will be well to con- 

 sider some questions more fully when discussing the structure of 

 true schists ; but I think it would be impossible to give a better 

 illustration of my meaning than the case of some concretionary 

 limestones. In these the facts seem to show very clearly that the 

 material was originally deposited in a pretty uniform condition; 

 but the carbonate of lime has collected in more or less lenticular 

 masses, or filled transverse fissures to form veins, and left a great 

 part of the non-calcareous deposit as a laminated shale wrapping 

 irregularly round the concretions. On the whole such a rock may 

 be truly stratified ; but the details are not true stratification. So, 

 in the case of these altered slates, the existing structure is due to 

 irregular segregation, controlled by the previously existing struc- 

 ture, whether it was stratification, close joints, or true cleavage. "We 

 might very well draw the same conclusion from the study of true 

 schists ; but then it would be, to a considerable extent, more a 

 matter of inference than of proof, whereas in the rocks now under 

 consideration both true stratification, close joints, true cleavage, 

 and true foliation are present to such an extent that their mutual 

 relations are easily seen. 



