74 Transactions. 



» 



2. Fracture Cleavage. 



This phenomenon, hitherto undescribed, is well displayed 

 in the lower schists at Alexandra and on the Dunstan Range. 

 Here we find a series of cross-fractures rilled with quartz, and 

 inclined to the foliation or flow-cleavage planes at an angle of 

 45° (Plates XI and XII). The veins thus formed are widely 

 spaced and discontinuous. 



This is a typical example of fracture cleavage developed by 

 shearing in the zone of rock-fracture. Its mode of origin has 

 been pointed out by C. R. van Hise, who says, " In the zone 

 of rock-fracture, where the differential stress surpasses the 

 ultimate strength of the rock, there may be produced a fissility 

 in two sets of intersecting planes equally inclined to the greatest 

 pressure."* In Otago one set is generally emphasized to the 

 exclusion of the other. The name " fracture cleavage " is due 

 to C. K. Leith, who has discussed its nature at length in his 

 monograph on " Rock-cleavage. "f 



In some of the upper members of the schists, shearing-planes 

 occur frequently along the foliation-planes, and there result 

 slip-bands marked by a line of crushed and broken rock. These 

 are well seen in some of the railway-cuttings in the Taieri Gorge. 



It thus appears that the effect of shearing - stress differs 

 according to the depth of the rocks affected, since fracture 

 cleavage in the lower schists gives place to slip-bands along the 

 foliation-planes in the upper schists. 



3. The Chlorite-schists. 



Distinctive types of chlorite- schist occur generally near the 

 base of the mica-schists, notably the coarse chlorite-schist on 

 the Dunstan and Pisa Ranges, and the granular chlorite-schist 

 in younger beds at Cowcliff Hill, near Gibbston. These have 

 all the characteristics of metamorphosed igneous rocks, an 

 origin which was suggested by Hutton for some bands of chlorite- 

 schist near Queenstown.J 



Field Relations. 



The two types referred to are interbedded with the mica- 

 schists, in bands varying from 50 ft. to 300 ft. thick. They 

 are frequently underlain by thin distinctive bands of micaceous 

 quartz-schist, which may represent altered contact rock. 



* C. R. van Hise, "Principles of North American Pre-Cambrian 

 Geology," U.S. Geol. Survey, L6th Annual Report, part i. p. (143. 



fC. K. Leith, " Rock-cleavage," Bulletin No. 239, U.S. Geol. 

 Survey (1905), p. 119. 



JF. W. Hutton, "The Foliated Rocks of Otago," Trans. N.Z. Inst.. 

 vol. xxiv (1891), p. 360. 



