F. P. Mennett — Basic Dykes and Rock Genesis. 13 



Under the microscope this is, however, seen to be far from the truth. 

 To the microscopic details of the specimens we may now turn. 



The Mixture Rocks. [1346.] This slice is taken from a rather 

 fine-grained rock nearer to Kahlele's Kraal than most of the others. 

 In the hand-specimen it appears a perfectly normal dolerite, consisting 

 of felspar and augite, and in some parts of the slice there is little 

 to disturb such a conclusion. However, there are several large 

 xenocrysts of felspar, one so nearly dissolved that without the 

 evidence of the others its true nature might not be suspected. These 

 felspars have an altered appearance, and a very peculiar mottling, 

 evidently due to the heating they have undergone, but the cross- 

 hatching of microcline is nevertheless recognizable. Two quartz 

 xenocrysts about 3 mm. long also occur. The most interesting feature 

 of the felspar xenocrysts is their recrystallization along their edges 

 into clear felspar, contrasting sharply with the cloudy interior. This 

 sometimes shows distinct crystal faces, and is idiomorphic towards 

 quartz and micropegmatite. The last is somewhat abundant, and in 

 view of the presence of xenoliths in all stages of solution, obviously 

 derived from the surrounding granite, there can be little doubt that 

 it results from the complete absorption of others, especially bearing in 

 mind the relation already noted to the latter growths of felspar. The 

 patches where micropegmatite is most abundant generally show some 

 larger grains of quartz, and are, moreover, characterized by the presence 

 of small biotite flakes instead of the augite of the normal doleritic 

 patches. 



PI. I, Fig. 3 [1701]. This represents a further stage of admixture. 

 In hand-specimens it is a black fine-grained rock enclosing many con- 

 spicuous xenoliths of granitic material. The ground-mass has under the 

 microscope a rather granular appearance (see Fig. 3), but is permeated 

 everywhere by fine micropegmatite. Some basic felspar remains, 

 but biotite is the ferromagnesian mineral. Most of the quartz 

 xenocrysts and some of those of felspar are composite. Beautiful 

 examples are to be seen of corrosion cavities in the felspars, filled with 

 micropegmatite, the clear felspar of the latter being in optical 

 continuity with the remainder of the xenocryst. This is illustrated 

 in Fig. 3, where two such cavities are shown in a crystal of oligoclase, 

 which has the characteristic cloudy mottled appearance of all the 

 felspar xenocrysts of this intrusion. 



Slice [1703] is similar to the above. It has a very interesting 

 feature in the presence of an orthite (allanite) crystal, which shows 

 as an oval dark patch at the edge of Fig. 4. Orthite is an almost 

 constant accessory in the Matopo granite, 1 and is well developed in 

 the rock round this particular intrusion. 2 Slice [1351] is also similar. 

 It has numerous examples of cloudy felspar xenocrysts extinguishing 

 simultaneously with the fresh felspar of the surrounding micro- 

 pegmatite, which last forms the bulk of the rock. One corrosion 

 cavity has an edging of fresh felspar with well-developed crystal 

 faces idiomorphic towards the enclosed micropegmatite. 



1 See Geology of Southern Rhodesia, 1904, p. 29. 



2 Geol. Mag., January, 1910, p. 18, and PI. V, Fig. 10. 



