ANN^rVEESAET ADDEESS OF THE PEESTDENT. 87 



origin as slates, since these specimens are probably due to the 

 alteration o£ slates which contained no sand. 



The evidence of the original fine-grained material is occasionally 

 tolerably satisfactory, but often equivocal. We must always be on 

 our guard not to mistake a portion of decomposed or altered fel- 

 spar for the original fine-grained base of a slate, and can place no 

 reliance on observations made with specimens of gneiss. I would 

 base my arguments wholly on what may be seen in true mica- 

 schist, in which there is no good evidence of the former presence 

 of crystalline felspar, either in broken fragments or as crystals 

 formed in situ. All the sections examined were prepared by my- 

 self, and no polishing material used ; so that I am quite sure that 

 none of the appearances is due to any thing of that kind sticking 

 to the objects. The best examples I have seen are from Skateraw 

 and Muchals, on the sea-coast between Aberdeen and Stonehaven. 

 In them I have not been able to find any independent proof of the 

 former presence of felspar, or of any other mineral likely to yield 

 fine-grained pseudomorphs. The patches which I take to be por- 

 tions of the slightly-altered original rock show no trace of crystal- 

 line form, and have no well-defined outline, but, on the contrary, 

 pass gradually into the surrounding schist. They have also a de- 

 cided general structure parallel to the well-marked stratification of 

 the rock, which would not be at all probable if they were not really 

 portions of material mechanically deposited. They are as fine- 

 grained as any slate, and correspond closely with certain light- 

 coloured slightly altered rocks met with in the mountains of North 

 Wales, not far south of Aber. On the whole, taking every thing 

 into consideration, the facts seem to indicate strongly that the 

 patches are not pseudomorphs, but those portions of an original 

 fine-grained rock which have not recrystallized into large crystals 

 of mica and quartz. This supposition, of course, agrees so well 

 with the evidence derived from the quartz-sand, that perhaps we 

 may adopt it provisionally, even though we might feel bound to 

 admit that, without independent evidence, the facts are insufficient 

 to justify such an important conclusion. A large proportion of my 

 sections of mica-schist furnish more or less distinct evidence of the 

 like kind, which, however, in most cases, may be said rather to 

 agree with the supposition that they were originally fine-grained 

 strata than to really prove it. As a general rule, all that can be 

 confidently asserted is, that the great bulk of the rock is composed 

 of minerals which have so crystallized in situ that the original struc- 

 ture has been almost or altogether obliterated. 



Original Source of iJie Material of some Schists. 



As shown above, there is sufficiently good proof that the schists 

 of the central Highlands of Scotland were originally slates ; and it 

 would obviously be interesting to know the nature of the rocks 

 from which their material was derived. There are, however, some 

 serious difficulties to contend with in deciding this question. The 



