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DR. A. HOLMES ON THE TERTIARY 



[vol. lxxii r 



solution containing soda and silica in water charged with carbon- 

 dioxide. 



In explaining the origin of amygdale minerals, appeal has been 

 made to two chief processes. By some writers they have been 

 regarded as decomposition-products due to weathering ; by others 

 they are regarded as hj'drothermal products of the final stages of 

 consolidation. The view is now gaining favour that these minerals 

 represent, not the first stage of destruction of the lavas in which 

 they occur, but the last stage in their construction. 1 C. N. Fenner 3 

 takes an intermediate position : for, although he concludes that 

 the vesicles of the Watchung basalt were filled while it cooled, he 

 thinks that the water concerned in the process was drawn from 

 the underlying sediments. 



In Mozambique the amygdale minerals are clearly not the 

 result of weathering. In the freshly exposed surfaces on the 

 Mochelia road and on the Ibrahimo road, weathering products 

 are typically absent. The felspars are generally fresh, and, where 

 altered, they are replaced by chalcedony and penetrated by- 

 streaks of chlorite — a change clearly to be correlated with the 

 replacement of heulandite by silica and chlorite. If due to atmo- 

 spheric weathering, the distribution of zeolites and agate should 

 be uniform : on the contrary, it is very irregular. In some 

 localities the basalts ai'e vesicular and free from infillings ; in 

 others the walls of the vesicles are lined to a varying depth, 

 while along certain belts the vesicles are all tightly packed. The 

 view suggested by Fenner cannot be adopted, for the amygdales 

 occur as abundantly when the basalts lie on a platform of granite 

 and gneiss as when they cover a sedimentary basement. There 

 seem to be, however, no ob]ections to regarding the minerals and 

 the water that accompanied them as the last products of consoli- 

 dation. The regular secmence which, so far as can be seen, is one 

 of decreasing temperature and of increasing concentration of soda 

 and silica; the instability of heulandite towards silica ; the absence 

 of zeolites from the sills and dykes (because the material that 

 would have formed zeolites was enabled to crystallize as plagio- 

 clase ) ; the general absence of calcite, limonite. and other 

 weathering products ; the conspicuous freshness of the pyrogenetic 

 minerals ; and, finally, the occurrence of hot springs on the border 

 of the volcanic field — all favour the hydrothermal or late 

 magmatic origin of the amygdale minerals. 



1 For a recent discussion and references to many other localities, see 

 W. P. P. McLintock ' On the Zeolites & Associated Minerals from the Tertiary 

 Lavas around Ben More, Mull ' Trans. Roy. Soc. Edin. vol. li, pt. 1 (1915) 

 p. 13. 



2 Ann. N.T. Acad. Sci. vol. xx (1910) p. 106. 



