214 Proceedings of the Royal Physical Society. 



sandstone into quartzite, and of shales and clays into Lydian 

 stone, near their contact with dykes and sills, is here 

 attributed to deposits of silica from the heated alkaline 

 waters which form the ''mother liquor" of the eruptive 

 rocks. 



Where the rock affected happens to be one of eruptive 

 origin, the new minerals developed always bear a definite 

 chemical relation to the constituents of the rock affected. 

 Amongst the minerals most commonly developed in rocks of 

 sub-basic composition by thermo-metamorphism are Garnets^ 

 and Epidote. In the case of an andesite in which the 

 vesicles once contained Saponite or Celadonite, these pass 

 into Epidote. Not uncommonly some part of the decom- 

 position products of the ferro-magnesian minerals may again 

 return to the form of their parent minerals, and Delessite 

 and its allies just referred to may redevelop into one of the 

 Amphiboles. In like manner some part of the zeolites may 

 go back again to felspar ; and Chalcedony, Opal, and other 

 colloidal and semi-colloidal forms of silica may go back 

 to Quartz. Eine examples of this latter change are shown 

 by the agates in Ordovician lavas of the English Lake District, 

 where they are pierced by granite masses ; and still better 

 in the Old Eed Andesites of the Ochils, where they come 

 near to intrusive masses like the Tillicoultry diorite. 

 Where the Arenig Eadiolarian Cherts come near to the 

 granites of Galloway, the colloidal silica is also converted 

 into granular Quartz. 



The second division (h), which comprises those minerals 

 resulting from hydrothermal action, in those cases where 

 some part of the constituents have been introduced from 

 without, form a connecting link with B 1. As an example, 

 not already referred to, may be mentioned Tourmaline, 

 Practically most of the remainder occurring in Scotland have 

 been referred to already ; but this may be the right place 

 to remark that some of the minerals already referred to as 

 generated in metamorphic limestone, may be partly due to 



^ The great majority of garnets appear to be due to the thermo-meta- 

 morphism of rocks which are primarily of eruptive origin. 



