﻿part 3] TERTIARY VOLCANIC ROCKS OF MOZAMBIQUE. 



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the shore-facies of the Oligocene deposits of Lumbo and Mozam- 

 bique Island (fig. 5). From this more detailed evidence, a lower 

 limit may be assigned to the period of vulcanicity, determining the 

 age of the vulcanism as late Oligocene or post-Oligocene. 



(c) Evidence from the Composition of the Oligocene 

 Limestones. 



In the Oligocene deposits of Mozambique Island and Lumbo, 

 fragments of Cretaceous limestone occur that may be easily 

 identified, although their size is small compared with the similar 

 contained fragments in the Mochelia conglomerate. Now, if the 

 conditions of denudation and deposition were such that pebbles of 

 limestone were preserved, it seems reasonable to suppose that, if 

 the amygdaloids had existed at that time, they would also have 

 played some part in building up the Oligocene formations. At the 

 present day, the rock-detritus that is distributed over the littoral 

 plains at high- water level, invariably contains large numbers of 

 agate- and jasper-pebbles ; and similar, though less abundant, relics 

 of igneous activity are found among the gravels of the numerous 

 raised beaches which fringe the shore from point to point. Hence, 

 since no trace of basalt nor of its siliceous amygdales have been 

 detected in any of the Tertiary sediments, one may conclude with 

 some confidence that the basalts had not then been extruded, and 

 consequently that the period of their extrusion was late Oligocene 

 -or, more probably, post-Oligocene. 



The phonolite and associated lavas from the Sanhuti district lie 

 across the boundary between Cretaceous and Archaean rocks. 

 Their age is, therefore, post-Cretaceous ; but their relations to the 

 far more abundant amygdaloids are as yet entirely unknown. 



VII. Order of Eruption, and Classification. 



The lavas from the Sanhuti mentioned above were collected by 

 Mr. Reid during a journey to Mount Meza. In the case of three of 

 his specimens, the order of eruption can be definitely stated. Near 

 the base of the volcanic series, lying on a floor hidden by lateritic 

 earth and gneissic debris, is a dark compact basalt containing 

 rusty amygdales partly occupied by calcite. Both in the com- 

 position of the amygdale minerals and in its microscopical aspect, 

 the rock differs conspicuously from all the other basalts of Mozam- 

 bique. At a higher level a lava of lighter colour was met with. 

 This is the phonolite, and it also contains calcite as the chief 

 occupant of the vesicles. At the top of the series a black 

 vesicular basalt was found. Owing to its extremely mafic character, 

 it is here designated a picrite -basalt (see § XIII, p. 244). The 

 specimens next collected include a fragment of amygdaloidal basalt 

 rich in chalcedony and heulandite. This rock is identical with the 

 amygdaloidal basalts occurring elsewhere in the volcanic area, and 

 was found in situ on a sedimentary basement. 



It is interesting to notice that, in Abyssinia, W. T. Blanford 

 found two equally contrasted series of lavas. The lower consisted 



