part 1] AXD ASSOCIATED ROCKS OF .MOZAMBIQUE. 



47 



groins of colourless unserpentinized forsterite are seen among the calcite- 

 plates, accompanied here and there by a few similar grains of colourless 

 diopside. Small flakes of graphite are sprinkled through the limestone, and 

 it is to these that the rock owes its grey coloration. 



At the south-eastern corner of the mass the limestone is bounded 

 by a dyke or vein of extraordinary composition, which lias inserted 

 itself between the limestone and the enclosing gneiss. The speci- 

 men collected (No. 74) is made up almost entirely of a felt of pale- 

 green muscovite in which there are granular streaks and rounded 

 or lenticular aggregates of rutile, accompanied in places by granular 

 intercalations of quartz (see p. 73 for a detailed description, and 

 PI. X, fig. 4, for a photomicrograph). 



In the immediate neighbourhood of the muscovite-rutile vein, 

 the limestone becomes much richer in silicate-minerals. Olivine 

 disappears, and gives place to diopside ; while muscovite becomes 

 abundant, and titaniferous minerals begin to appear. 



No. 73, collected between the muscovite-rutile vein and the purer lime- 

 stone, consists mainly of calcite and granular aggregates of colourless 

 diopside, with a considerable proportion of muscovite varying in colour from 

 white to pale green. A small amount of interstitial quartz is present, 

 and associated with the mica are occasional grains of rutile and larger 

 masses of sphene. 



The following table gives the mineral compositions of the 

 crystalline limestone and the muscovite-rutile vein, for comparison 

 with that of the contact-rock produced by their interaction. 



Table III. 



Minerals. 



Crystalline 



Limestone. 



No. 72. 



Contact- 

 Rock. 

 ■ No. 73. 



Jfuscovite- 



Rutile Vein. 



No. 74. 



Calcite 



80 



12 



8 



36 



18 ' 



8 



2 

 33 



3 





Forsterite 



Diopside 



— 



Sphene 



Rutile 



27 



Muscovite 



■ 

 68 



Quartz 









On the eastern side of the Monapo Limestone the latter is cut 

 by small intrusions of coarse-grained pegmatite containing per- 

 thitic orthoclase, oligoclase, and biotite as its chief constituents. 

 In the specimens collected very little quartz is present. Imme- 

 diately within the contact occur beautifully bladed crvstals of 

 actinolite, averaging an inch in length. Inside this zone is a 

 band 3 or 4 inches wide composed of a fine-grained aggregate 

 of tremolite and actinolite with intersertal felspar and calcite 

 (No. 91). In a neighbouring locality the limestone has been cut 

 by small intrusions of granite containing much quartz and a 

 little biotite ; and, in this case, diopside has been produced as 

 the product of interaction in place of actinolite (No. 92). 



