8 HOLLAND : PECULIAR FORM OF ALTERED PERIDOT1TE 



and altered soon after their formation by the vapours originally 

 included in the magma and excluded to the mother-liquor, or they may 

 become attacked at a distinct and subsequent period. In the former 

 case the processes of primary crystallization and secondary alteration 

 are continuous and in reality phases of the same process. This 

 explanation is not difficult to accept when the alteration is very 

 limited in extent, as for instance, when the aqueo-igneous mother- 

 liquor in a diabase-magma attacks the early-formed augite and 

 changes it into biotite with concomitant excretion of magnetite. 1 



The extension of this idea to a complete alteration of the whole 

 rock is a step one naturally hesitates to take, and, on account of the 

 absence of recognisable stages in the process, one which is less easily 

 proved. It is in this case, nevertheless, an explanation more satis- 

 factory than either of the two alternative theories, namely, (a) 

 primary consolidation of a dunite magma containing water and 

 carbonic acid, or [b) alteration of a dunite after its consolidation by 

 the secondary action of water and carbonic acid. 



There is no doubt that the action of vapours originally contained 

 in a magma has often been overlooked, and minerals like the zeolites 

 epidote and serpentine have been thoughtlessly referred to secondary 

 and independent processes such as weathering. We hope we are not 

 now exaggerating the importance of these vapours in going to the 

 other extreme and regarding the remarkable rock described above 

 as the result of the complete alteration of the dunite by the vapours 

 contained in its own magma. There is but a step from this conclu- 

 sion to the idea of primary consolidation of an aqueo-igneous dunite 

 magma charged with carbonic acid. For this last step, however, there 

 is at present no direct support. To take it would involve the recog- 

 nition of the water and carbonic acid as essential constituents and 

 necessitate the distinction of the rock as a new type in the category 

 of igneous species. Had the alteration taken place as a distinctly- 



1 Cf. Holland : Quart. Juurn. Geol. Soc, Vol. 53 (1897), p. 405 ; Geo!. Mag., 

 December 1899, p. 543. 

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