12 F. P. Mennell — Basic Dykes and Rock Genesis. 



edges. The doleritic material is composed of the usual lath-shaped 

 felspars, but, as in the previous case, the ferromagnesian mineral is 

 now represented by hornblende. Between the quartz and felspar 

 grains there is always a reaction rim, usually about -iV Tam. broad. 

 In some instances this has consolidated as a fine micropegmatite, 

 a most important observation in view of the phenomena to be described 

 below. 



The Dyke near Kahleles. 'We may now pass on to the consideration 

 of another intrusion near the central part of the Matopo granite mass. 

 It is a much larger one than that furnishing the specimens previously 

 referred to, running for several miles in a north and south direction 

 through Kahlele's Old Kraal. The granite is here remarkably 

 uniform and of the normal type prevalent throughout the Matopos, 

 consisting of quartz, microcline, oligoclase, and biotite, with sphene, 

 apatite, and orthite as accessories. The width of the dolerite dyke 

 usually exceeds 100 feet, and it forms the bottom of a big depression 

 with steep granite kopjes rising on each side to heights of several 

 hundred feet. The tract from Fort Usher to the Blanket Mine follows 

 this depression along the whole length of the dyke. 



Microscopic Features. The normal coarser central portions of the 

 intrusions [1352 and 1700], as seen along the road north of Kahlele's, 

 afford examples of rather an unusual type of dolerite. Under the 

 microscope the rock is seen to consist of olivine, enstatite, augite, and 

 felspar in roughly equal proportions. The olivine occurs partly in 

 good-sized grains showing a tendency to crystal outline, and partly as 

 inclusions in the pyroxenes. It is very fresh ; some lines of separated 

 magnetite can be observed, but there is little serpentinization. The 

 enstatite and augite occur in rather large plates. Both are quite 

 colourless, and are apt to be penetrated to some extent by the felspars. 

 These generally show pericline as well as albite lamellation, but are 

 not very abundant, and the rock approaches the picrites in this 

 respect. Small flakes of a colourless to deep-brown mica may be 

 observed among the felspars. It may be noted here that further to 

 the south slices show that the dyke has undergone considerable 

 alteration. The ferromagnesian minerals are uralitized, and the 

 production of much epidote may be observed in many places. The 

 alteration was therefore due to deep-seated hydrothermal agencies, 

 and not to weathering. 1 At the point we are immediately concerned 

 with the rock is remarkably fresh, as in the specimens described 

 above, having a black sparkling appearance in hand-specimens. As 

 one approaches the granite margin, its texture becomes much finer, 

 and especially towards the west side xenoliths become of frequent 

 occurrence. They first of all appear as more or less rounded or 

 corroded patches of quartz or felspar, or both, in the black fine- 

 grained dolerite, but the rock passes into a type with a much lighter 

 ground-mass, as if a considerable amount of the granitic material had 

 been assimilated, as there can be no doubt was the case. Eventually 

 we encounter a rock which may be regarded as contact-altered granite, 

 and which does not look in the field as if it had been much modified. 



1 Compare Introduction to Petrology, p. 177. 



