﻿part 3] 



VOLCANIC ROCKS OF MOZAMBIQUE. 



251 



specimens are microporphyritic, and the smaller crystals are 

 frequently grouped together in glomeroporphyritic clusters. 

 Where individuals of markedly different size come into contact in 

 parallel positions, the Becke test indicates that the larger crystals 

 possess a slightly higher refractive index than their smaller 

 neighbours. This evidence, implying that crystals of the second 

 crop are slightly richer in the albite-molecule than those of the 

 first, is interesting, in view of the fact that the zeolites of the 

 amygdales are predominantly lime-bearing varieties. Some of the 

 larger crystals of labradorite — especially in the neighbourhood of 

 well-filled amygdales — are replaced by chalcedony, and veined and 

 streaked with chlorite. Signs of weathering, however, are rarely 

 met with. 



Augite is present in colourless or yellowish-green grains and 

 aggregates, the colour tending to deepen towards the outer borders, 

 which are often ill- defined and merge imperceptibly into the 

 surrounding glass. In the dark reddish- or greenish-brown glass 

 that occupies the interstices of the crystal network, minute 

 dendritic sprays of magnetite and innumerable globulites are 

 present, particularly around the borders of ill-defined felspar- and 

 augite-crystals. It is worthy of notice that the glass around the 

 larger vesicles is frequently very dense and undifferentiated. This 

 effect is doubtless due to the local cooling set up by the sudden 

 expansion of the volatile constituents of the erupted magma. 



Only the amygdale minerals remain to be considered. The 

 vesicles in which they occur ' are of two strongly-contrasted 

 types :— 



(a) Well-shaped and smoothly-lined spaces easily visible to the naked eye 



in thin sections (PI. XXI, fig. 4), and in hand-specimens extending 1 

 in dimensions up to the size of the closed fist. 



(b) Minute branching interspaces with obscure and irregular borders, 



connecting sometimes with the larger vesicles, and sometimes with 

 small cracks and fissures that cut abruptly across the texture of the 

 rock. 



The minerals found in the amygdales are chlorite, heulandite,. 

 laumontite, natrolite, opal, chalcedony, quartz, calcite, and gypsum. 

 In good exposures, such as that on the Mochelia road, it can be 

 seen almost at a glance that zeolites are by far the most abundant 

 of the amygdale minerals, and that of the zeolites heulandite is 

 the chief member. In the associated surface-debris, the zeolites 

 must soon fall a prey to decomposition, and consequently forms of 

 silica greatly predominate, giving the erroneous impression that 

 the parent amygdales were mainly siliceous. In hand-specimens 

 the order of infilling is found to be (1) chlorite (which appears 

 also in later stages), (2) zeolites, and (3) silica. 



Palagonite sometimes occurs in minute vesicles; but, with the 

 exception that it is occasionally replaced by chalcedony, it is. 

 unaccompanied by zeolites or other minerals. Chlorite is the 

 usual lining material, and in vermicular aggregates it is often 

 present in heulandite and chalcedony. 



