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VOLCANIC ROCKS OF MOZAMBIQUE. 



253 



difference, which may be of great significance,, between- the vesicle- 

 minerals of the Sanhuti lavas and those of the amygdaloids. On 

 the one hand, is a series of lavas including types rich in alkalies 

 in which soda-zeolites and calcite occupy the vesicles ; on the 

 other, is a series of typically calc -alkali lavas in which the 

 vesicles are occupied by lime-zeolites and forms of silica. These 

 two associations, which can be matched in other regions, seem to 

 indicate that among the volatile fluxes of the alkali 

 series carbon dioxide was a dominant member, whereas 

 among those of the calc-alkali series water was the chief 

 constituent, carbon dioxide being absent, or of minor importance. 



The order of infilling is closely parallel to that determined in 

 many other localities where the same minerals are represented. 

 As long ago as 1819 Breithaupt 1 recognized the sequence from 

 heulandite to stilbite (given in the original nomenclature as 

 stilbit to desmin). In the Watching basalt, New Jersey, 

 C. N. Fenner 3 shows that heulandite may be followed by laumontite, 

 stilbite, or natrolite. At the well-known locality of Carnmoney 

 Neck (Antrim), the order of infilling is given by J. Strachan 3 as 

 (a) hullite and dellesite, (b) zeolites, (c) chalcedony and quartz. 



The circulating solutions of the Mozambique amygdaloids 

 appear to have become saturated in alkaline constituents only 

 towards the end of the period of zeolite deposition. C. Dcelter has 

 shown 4 that natrolite is deposited from solutions of soda, alumina, 

 silica, and water at temperatures below 190° C. Above that 

 temperature analcime is formed. The presence of gypsum among 

 the end-products gives another point, on the temperature-scale. 

 Van't Hoff 5 investigated the relations of gypsum and anhydrite, 

 and found that the latter mineral is deposited i« place of gypsum 

 if the temperature exceeds 63'5° C. In the presence of alkali 

 salts the temperature of the transition -point is lowered. The 

 stage of silica deposition falls between those of natrolite and 

 gypsum, and at such low temperatures it is difficult to understand 

 why the silica was not all deposited as opal, unless it is assumed 

 that alkalies were present. Gr. Spezia 6 has shown that opal is 

 deposited at moderately low temperatures from siliceous solutions, 

 but that, when alkalies are present, quartz forms instead. Opal 

 already deposited may be transformed into quartz by the action 

 of a hot solution of sodium silicate. The order commonly observed 

 in siliceous amygdales (opal — chalcedony — quartz) seems, there- 

 fore, to imply a growing concentration of soda in the percolating 

 waters. Interesting in this connection is the composition of the 

 hot springs at Nashologoto, for they may be regarded as a feeble 



1 'Die Paragenesis der Mineralien ' Freiberg, 1849, p. 105. 



2 Ann. N.T. Acad. Sci. vol. xx (1910) p. 169. .... . 



3 Proc. Belfast Nat. Field Club, vol. ii (1906) App. vii & viii. 



4 Tschermak's Min. Petr. Mitth. vol. xxv (1906) pp. 99, 102.- 



5 Zeitschr. Phys. Chem. vol. xlv (1903) p. 257. 



6 Atti Aecad. Sci. Torino, vol. xxxiii (1898) pp. 289, 876, & Journ, Chem. 

 Soc. vol. lxxvi, pt. 2 (1899) p. 300 (abstract). 



