460 G. E. Tilley — AletamorpJnstn of 



H and K, in the pUogopite constitution, we may represent its 

 formation by tlie succeeding equation 



9 CaMg (COg)^ + H2KAI3 (SiOJs + 6 SiO^ + 3 H^O 

 dolomite sericite quartz 



= 2 H,Mg,Al (SiOJ, + 9 CaCOa + 9 CO^ 



H,KMg,Al_(SiOj3 



pLilogopite calcite 



H3^drofluoric acid being regarded as an essential constituent of 

 phlogopite, this will replace some of the water in the left-hand side 

 of the equation, entering the phlogopite molecule as the univalent 

 radicle, MgF, isomorphous with H and K. 



Whatever equation is adduced for the formation of phlogopite, 

 calcite is inevitably a by-product of the reaction, and phlogopite 

 must therefore be added to the group of minerals which induce by 

 their formation dedolomitization. 



Apatite is doubtless derived by the recrystallization of original 

 phosphate substance in the sediment, possibly of organic origin or 

 of detrital apatite of pre-existing rocks. 



The apatite is observed to be plentiful in those rocks in which 

 phlogopite is most abundant, and this association may have been 

 accentuated through the agency of fluorine. 



The presence of calcite as a constituent of decomposed forsterite 

 grains calls for some remark. 



The changes which have operated since the formation of forsterite 

 are firstly its replacement by seriDentine, with or without the 

 separation of magnetite, and this may be represented by the 

 equation 



2 Mg,SiO, + 2 H,0 + CO, = H,Mg3Si,0g + MgC03_ 



The magnesium carbonate is either removed in solution or is 

 absorbed into the calcite or dolomite molecule during this change. 

 The change has in ma^y cases proceeded further, however, and in 

 the dolomites of Sleaford Bay serpentine has yielded pseudomorphs 

 of opal and calcite. 



H^MgaSi^Og + 3 CO2 = 2 SiOa + 3 MgCOg + 2 H^O 

 opal 



Here, again, magne'site is released, and although this may be 

 present in some of the pseudomorphs, yet in the majority its place 

 is taken by calcite. 



The origin of the magnesite so characteristically associated with 

 the graphite schists in the vicinity, and developed in large masses 

 in the northern area, and essentially of secondary origin, can probably 

 be traced in part to those reactions which have ultimately resulted 

 in the degradation of forsterite to opal and calcite. 



The conversion of the compact serpentine into the fibrous 

 chrysotile asbestos is apparently a recrystallization in situ, 

 under conditions of non-uniform pressure, in which case the veins 

 represent potential fractures, the elongation of the resulting 

 chrysotile fibres being normal to the direction of maximum pressure. 



