360 F. P. MemieU—The Rhodesian Banket Beds. 



Museum Eeport for 1902, Since then further investigation has 

 shown how widespread is their distribution throughout the country, 

 and has yielded much evidence as to their origin, lithological 

 characters, and stratigraphical position, the discovery of gold in the 

 Loniagundi district having been instrumental in promoting an active 

 search for the rock in neai-ly all the mining districts. 



In view of the fact that specimens have been submitted to experts 

 outside the country, who have been disposed to favour the idea that 

 the rock is a ' crush -conglomerate ' rather than an altered sediment, 

 it may be as well to at once state the evidence against such a view. 

 The internal evidence of even a good series of specimens is quite 

 sufficient to negative the idea, while no one fully acquainted with 

 the geological features of Ehodesia would ever have given it 

 consideration. 



As an Archaean rock, the conglomerate has undergone a considerable 

 amount of alteration. It exhibits the results of shearing and crushing 

 to a marked degree, as well as changes due to the infiltration of 

 secondary minerals. But at the same time its original features are 

 by no means unrecognisable. Thus we find that there is no grading 

 of the pebbles into the surrounding matrix : in spite of crushing and 

 slips they show perfectly well-defined boundaries even in sections 

 under the microscope. More than this, they exhibit the greatest 

 variety of rock types, and often, one may say as a rule, the common 

 types of pebble are quite different in composition to the matrix. 

 Thus, even where quartz and granite fragments are predominant, 

 there is often no quartz in the finer material. There are frequently 

 scarcely any repi-esentatives of the surrounding rocks among the 

 pebbles, which comprise on the other hand petrological types 

 nowhere known in Rhodesia at the pi'esent day in situ. I may 

 instance the fact that a few yards of outcrop near Hopefountain 

 yielded me on a hurried visit the following varieties of pebble : 

 white glassy quartz, dark quartz, banded ironstone, ' halleflinta,' two 

 kinds of chlorite schist, hornblende schist, granite, grauophyre, 

 porphyrite, and at least two distinct kinds of amygdaloidal lava. 

 It will be evident from this that the crush theory is by no means 

 strengthened even by such a supposition as the entire breaking down 

 of an igneous intrusion. But it is the stratigraphical evidence which 

 is decisive, and it is unnecessary to dwell on the points already 

 raised in view of the distribution of the conglomerate over the entire 

 country, taken together with its great thickness and its position 

 being always reconcileable with its occupying a constant horizon. 

 The rock is, in fine, precisely what one would expect to result from 

 the denudation of an extensive area of varied igneous and metamorphic 

 rocks. Hard and resistant substances like vein quartz, banded iron- 

 stone, granophyre, and halleflinta form most of the pebbles, while 

 the more easily disintegrated granites and schists have usually gone 

 to make up the finer matrix. The ready decomposition of the more 

 basic igneous rocks and hornblende or chlorite schists affords an 

 obvious explanation of the cases where the pebbles are largely 

 quartz, with some granite or granophyre, in a matrix now converted 



