196 Messrs. Chalmers 8f Hatch — 3£ashonaIand 8f Mataheleland. 



composing the metamorphic series are cliiefly chlorite and horn- 

 blende schists, with which are associated epidiorites, actinolite rock, 

 diabases, dolerites, and other basic igneous rocks. The intimate 

 association of basic igneous rocks with the schists was a fact we 

 have had ample opportunity for noting, and our observations lead to 

 the conclusion that the schists have been derived in great measure 

 fi'om those rocks, and that their foliated or schistose structure has 

 been produced by a mechanical process analogous to "shearing" 

 and referable to certain movements of the earth's crust along zones 

 of fracture or weakness.^ 



The sequence of events in the formation of the schists would be 

 somewhat as follows : — The granite crust acted upon by enormous 

 lateral pressure and tension produced by shrinkage, folding, and 

 rucking would tend to develop main lines or zones of strain, 

 weakness, and fracture, through which the molten basic magmas 

 hitherto pent up within the earth's crust would be forced up to form 

 dykes and iuti'usive masses of igneous rock along these zones. The 

 earth-movements, continuing along the main lines of fracture, would 

 effect the mechanical metamorphism of these rocks to schist, and 

 perhaps simultaneously cause the formation of open fissures ; and 

 these, offering a free passage to mineralizing solutions, would give 

 rise to the quartz lodes that form the auriferous deposits of the 

 country. It is reasonable to suppose that a portion of the granite 

 would also be involved in the zone of movement ; and indeed we find 

 evidence of this in the fact that gneiss or foliated granite is 

 generally found on the margin of a schist belt or within the belt 

 itself. 



Transitions from an unaltered igneous rock to a highly foliated 

 unctuous schist are clearlj^ discernible in many parts of the gold 

 belt. The minute structural and mineralogical changes involved in 

 the transition can be best studied with the aid of a microscope. 

 Starting, say, with a dolerite or diabase, the gradation of change 

 will be roughly as follows : — Dolerite being a rock composed of 

 plagioolase felspar and augite, the first step is the conversion of the 

 augite to hornblende. This is one of the most common changes 

 in the vvdiole range of petrology. Augite and hornblende are 

 practically identical in chemical composition, but have a different 

 crystalline structure. Hornblende is, however, more stable than 

 augite at ordinary temperatures ; consequently the first results of 

 molecular rearrangement in rocks containing augite is the para- 

 morphism of that mineral to hornblende. The rock produced by 

 this joint change consists therefore of plagioclase felspar and 

 secondary hornblende, and is known as an epidiorite. 



The next change is a breaking down of the felsjoar, producing 

 a granular aggregate of quartz and reconstructed felspar, and a 

 rolling out of the hornblende into ribbons or folds. This process, 

 known as foliation, produces a hornblende schist. The meta- 

 morphism progressing, the hornblende becomes replaced by chlorite, 



1 See W. Gibson, Quart. Jouni. Geol. Soc, vol. xlyiii, p. 404 ; also A. E. Sawyer, 

 ibid. vol. 1, p. 144. 



