212 Pi^oceedings of the Royal Physical Society. 



B 5. With regard to those minerals that arise from Thermo- 

 metamorphism, or, rather, through metamorphic changes set 

 up by Hydrothermal action, the case is very different. A 

 large number of the most important of the Scottish minerals 

 are due directly, or indirectly, to this cause. 



These may be divided into two sections : in the one the 

 constituents of the minerals resulting from the change were 

 already in existence in some form or other within the rock 

 in which they now occur ; in the other case, one or more of 

 the constituents have been introduced from some source 

 exterior to the present nidus of the minerals in question. 



The first section may conveniently be subdivided, in 

 accordance with the nature of the predominant constituent 

 of the rock affected, as follows : — 



If that predominant constituent is calcareous — the lime 

 being associated with impurities consisting of various other 

 substances — these latter, after the rock has been long exposed 

 to the solvent action of water at high temperatures, and, also, 

 presumedly, under considerable pressure, may eventually 

 separate out in more -or -less crystalline, and often idio- 

 morphic forms, when these conditions of temperature and 

 pressure change. A great variety of minerals may thus 

 arise. In Scotland, the commonest minerals developed by 

 this molecular rearrangement are Idocrase, Cinnamonstone 

 (and, occasionally, Grossular, and in a few instances one 

 or two other forms of Garnet), Sahlite and its varieties, 

 Tremolite, Wollastonite (but not Pectolite), Zoisite, Apatite, 

 Sphene, Andesine (but not Anorthite), normal Biotite (but 

 not Phlogopite in Scotland), Muscovite, Pyrrhotite, Spinel, 

 Talc, and Porsterite. Serpentine, which is found in meta- 

 morphic limestones, is probably a secondary product of some 

 ferro-magnesian mineral. The occurrence of these minerals 

 is strongly suggestive of the conclusion that the original 

 limestone has slowly undergone solution and subsequent 

 recrystallisation in sit4. Other facts seem to confirm this. 



In the cases where alumina predominates in the rock the 

 same heated waters may bring about a partial solution of 

 some of the constituents, and a subsequent molecular re- 



