ANNIYBKSARY ADDRESS OF THE PEESIDEIfT. 77 



The slate of Birnam closely corresponds with that of Penrhyn, 

 and is mainly an abnormal fine-grained micaceous deposit. 



Some of the rocks in the district of Moffat are of the usual 

 granular type ; but others are highly micaceous, and in one speci- 

 men I have found a little pumice and what appear to have been 

 fragments of glassy fibres like the so-called " Pele's hair." This 

 glassy material constitutes, however, only 3 or 4 per cent, of the 

 volume of the rock. 



Passing to the English Lake-district, the slate of Portinscale is of 

 the normal granular type ; but rocks of very similar age not far 

 from the granite on Skiddaw Forest are, as is well known, greatly 

 altered. The original material is almost wholly changed into 

 crystalline minerals developed in situ, amongst which hornblende 

 and chiastolite are the most characteristic. 



Near Windermere the slates above the Ooniston Limestone are 

 also of the normal granular type ; but possibly there are considerable 

 local variations, since the pencil-slate of Shap is almost wholly 

 micaceous. 



As already mentioned, the slate-rocks in North "Wales similarly 

 vary very greatly in different beds and localities ; and, as far as I 

 have been able to ascertain, the same remark will apply to those of 

 Devon and Cornwall. 



We may thus, I think, consider it proved that the original 

 mineral composition of our older stratified rocks varied greatly, 

 though much remains to be learned respecting the horizontal and 

 vertical distribution of the different types. Even when they really 

 differ much in constitution, the general appearance of the finer- 

 grained varieties is not strikingly different ; and the presence or 

 absence of cleavage still further obscures the difference. An ob- 

 server might thus easily overlook variations which might be of very 

 great importance in connexion with metamorphism. It appears to 

 me that the development or absence of certain minerals in meta- 

 morphic rocks depended as much on the original nature of the 

 material as on any mere difference of the temperature to which 

 the rocks have been exposed ; so that lower-lying rocks might very 

 well be less altered than those above them, even though probably 

 at one time exposed to a higher temperature, invading them from 

 below. 



General Changes after Deposition. 



The development of such minerals as pyrites in stratified rocks 

 is so well understood that I need not occupy time in saying any 

 thing about it, and will confine my remarks to what may be called 

 stony minerals. 



If a stratified rock consisted of material which was thoroughly 

 decomposed by weathering into stable products, it is clear that, 

 when further exposed to water and carbonic acid, no considerable 

 change could occur at the normal temperature, though it might be 

 consolidated by the introduction of soluble minerals. On the con- 

 trary, if the constituents of the deposit were in a state of unstable 



