258 F. P. Mennell — Archcsan StraUgrajthy. 



Definite evidence of the relative ages of different formations is not 

 often easy to obtain. Even if the dips could be relied upon, they 

 are usually vertical or nearly so. There is one point, however, to 

 vphich attention may be drawn. A little consideration will show 

 that the great granite masses which are so important a feature of 

 these areas must, as a rule, indicate anticlinal or dome -like 

 structures, and we find in fact that where the dips depart from the 

 vertical they are nearly always away from the granites. Where 

 this is the case the relative distances of the different rocks from the 

 granites may be taken as one of the chief aids in determining the 

 succession. For it stands to reason that the oldest beds will be 

 nearest the axis of elevation marked by the granite. Any indication, 

 however, of marked transgression on the part of the igneous rock, 

 such as dip of the foliation towards the latter, or the strike of the 

 schists being directly into it, prevents such a criterion being relied 

 on. I may say, however, that even where the dips are reversed 

 the rocks generally occur in their normal succession, as determined 

 elsewhere. An important point to remember is that metamorphosed 

 igneous intrusions figuring amongst the schists may behave as rocks 

 of any date earlier than their real period of formation. This follows 

 naturally from their protrusion from below, and hence it is very 

 difficult in some cases to distinguish between them and the much 

 older rocks on which some of the schists of sedimentary origin (in 

 which they are intrusive) have been laid down. Their intrusive 

 nature has to be inferred from (1) petrological characters, (2) the 

 way in which they make unexpected appearances among the old 

 sediments, (3) such traces as may survive of contact action produced 

 by them, (4) absence of pebbles derived from them in such 

 conglomeratic beds as may occur. 



The last reference to conglomerates reminds us of what an 

 important feature they may be in Archaean stratigraphy. Very 

 few developments of Archaean rocks fail to show important 

 conglomerate beds, and they are simply invaluable as datum-lines. 

 Not only are they readily recognised in the field as a rule, but their 

 pebbles afford most important evidence of their age. Thus the 

 relative ages of the two most important sedimentary series in 

 Rhodesia is at once determined by the fact that one of them, the 

 Conglomeratic series as it may be called, contains numerous pebbles 

 of the other, which I have termed the Banded Ironstone series. 

 The two sets of beds, though of enormous thickness, are folded 

 together in the most intricate mannei*, and though their order of 

 superposition was originally determined by me solely on the 

 strength of the evidence afforded by their general relative distance 

 from the granite, it is very satisfactory to get the additional proof 

 given by the pebbles. 



Even where the granites are transgressive in a marked degree, 

 evidence of the order of successicm of the metamorphic rocks may 

 sometimes be obtained from carefully considering their relations to 

 the granite. We may take an actual instance from the neighbour- 

 hood of Bulawayo. The figure, representing an area about 18 miles 



