184 Proceedings of the Royal Physical Society. 



but it is due in other cases to solfataric action (B 3) ; and 

 in others again, to pseudomorphous change (A 3). So with 

 Muscovite (to take an example of a different kind). This, 

 in Scotland, sometimes occurs as an original constituent of 

 acid eruptive rocks, and is therefore placed under B 1. More 

 often in Scotland it is a paramorph of Orthoclase felspar, 

 and therefore comes also under B 6. Such cases as these 

 are useful in bringing the geological side of mineralogy into 

 due prominence ; and they serve equally well to remind the 

 geologist of his dependence in such matters upon the work 

 of the chemist. 



I. Epigenic Minerals. 



A 1 (a and h). Under the first and second categories there 

 are Scottish minerals whose genesis calls for special remark. 

 Vivianite has been found in bogs in Shetland and elsewhere. 

 It appears to be generally due to a combination of the 

 phosphoric acid, derived from bones, with the iron usually 

 present in the lower part of peat bogs. There exists some 

 doubt whether crystallised specimens have yet really been 

 found in Scotland. Limonite and its allies are due to the 

 solvent action of surface-waters containing the humus acids 

 and carbonic acid upon various other iron-compounds, 

 whereby solutions of ferrous carbonate are formed. From 

 these, partly through the action of organic matter, the ferric 

 hydrate is formed. Some manganese compounds arise in a 

 similar manner. 



Al (c). Deposits formed in closed bodies of water — in 

 many cases under desert conditions. The principal Scottish 

 minerals formed under these conditions are Kock Salt, 

 Gypsum, Dolomite, Haematite, one or two of the ores of 

 manganese, and some concretionary forms of carbonate of 

 lime. In the case of Rock Salt, the primary sources of the 

 mineral may be fourfold. The salt may be simply washed 

 out of some rocks in which it happens to occur ; it may arise 

 through the direct combination of sodium salts and others 

 which yield hydrochloric acid; or it may be due to the 

 evaporation of shallow pools of sea-water in areas tempo- 



