260 F. P. Mennell — Archcean Stratigraphy. 



westwai'Ll. The Hillside intrusion has obviously risen by eating its 

 way upward, and not primarily owing to the folding of the schists, 

 which are not much thrown out of their course in its neighbourhood. 

 But the first impulse must all the same have been due to an incipient 

 fold, and though it is so slight it aifords an important clue to the 

 structure of the area. The apparent dip of the beds is vertical, as 

 near as may be, but though this may represent the position on 

 a small scale the general dip (or the true dip) of the beds cannot be 

 very steep, as may be inferred from the diagram. For slight as has 

 been the disturbance of the beds, the tilting has been sufficient to 

 prevent the conglomerate from anywhere coming into actual contact 

 with the intrusion on the north or east. It is evident from this, 

 first, that the Banded Ironstone is the older rock, and further, since 

 denudation has entirely removed the conglomerate from its surface, 

 even though the uplift has been so slight, that the folds cannot be 

 very sharp. If they were, the conglomerate would not wedge out 

 in the way it does before coming close to the intrusion. On the 

 south side of the intrusion there is probably extensive faulting where 

 the two systems of folds meet, for the Basement schists run as 

 a wedge into the conglomerate, without any intervening Banded 

 Ironstone. This will account for the difi"erence in structure that 

 is shown. 



A point of considerable importance in all Archaean areas is the 

 relation of schists and granite. As Professor Bonney points out, the 

 cases of gradation from mica-schist through gneiss into granite are 

 due merely to the fact that a kind of mica-schist is produced through 

 the crushing of gneiss, and the latter itself from the granite.^ But 

 even this is a rare phenomenon, for true mica-schists are not at 

 all common in my experience, and I know of hardly any so-called 

 gneiss which is not obviously an igneous rock — nearly always 

 a granite which has become somewhat ' streaky ' before complete 

 consolidation. To the unaccustomed eye, however, it is not at first 

 by any means a simple matter to determine whether a granite is 

 intrusive or not. I may say that though it was perfectly obvious 

 from the undisturbed condition of the granite, and the much folded 

 state of tl)e surrounding districts near Bulawayo, that the former 

 must be intrusive, I was a long time in bringing to light unquestion- 

 able evidence on the point. Indeed, if it had not been for a lucky 

 chance taking me over the magnificent contact section of the Matopo 

 granite, in an out-of-the-way locality near Figtree, it might have 

 been two years instead of only one before I was able to interpret 

 the evidence near Bulawayo. For the evidence is precisely' what 

 has led many geologists into recording instances of supposed 

 gradation between schists and granite, or, on the other hand, into 

 denying the intrusive character of the granite. Dr. Hatch, for 

 example, after making a tour through Khodesia,- actually came to 

 the conclusion that the schists were intrusions in the granite (!), 

 though he has since retracted this opinion in consequence of the 



' Proc. Geol. Assoc, vol. xv, p. 4. 

 '^ Geol. Mag., 1895, pp. 193, etc. 



