216 Proceedings of the Royal Physical Society. 



Ocliils, show this metamorphic change very well. The 

 feature is equally well exhibited by the tuffs of the same age 

 (Devonian) which occur under similar conditions in Lanark- 

 shire. Those of the English Lake District which have been 

 thus affected belong to two very different types, which may 

 be classed according to whether their alteration has been 

 effected contemporaneously (as it has been in most cases), 

 or whether the alteration has taken place long subsequently, 

 as in the case where those Ordovician volcanic rocks have 

 been invaded by granite masses during Devonian times. 

 The distinction is of much geological importance, but it 

 appears to have been previously overlooked. In the former 

 case the rocks may gradually change in sitil into holo- 

 crystalline rocks, in the latter, typified by the tuffs in the 

 contact zones around the Shap Granite, this reversion to a 

 crystalline character is much less likely to take place. 



B 6. Minerals which arise through Dynamic Meta- 

 morphism. 



The exact nature of some of the processes by which the 

 minerals referable to this category have been formed must 

 always remain more or less doubtful, because they have 

 certainly come into existence under conditions which it is 

 impossible to imitate experimentally. Still, evidence obtained 

 from a study of their mode of occurrence in the field, throws 

 at least some light upon their history ; while a careful 

 comparison of a large series of hand specimens, such as are 

 exhibited in the Edinburgh Museum, enables us to confirm 

 and extend the conclusions arrived at after a study of the 

 rock-masses on the larger scale in the field. 



Like some of the minerals previously referred to, these 

 under notice appear naturally to group themselves under 

 two leading categories. Under the first range those minerals 

 the whole of whose constituents, there is reason to believe, 

 were already in existence within the minerals which formed 

 the parent source. The second embraces those minerals 

 which appear to have arisen partly through the introduction 

 of one or more constituents from a source outside. 



