72' president's address — section c. 



Shag Valley (McKay, 1884), and especially in the central por- 

 tion of Northeim Otagc. Cotton (1917) has shown that there are 

 here broad areas 'made up of a complex cf schist and greywacjse, 

 so distinct in lithological characters as to determine distinctive 

 types of physiographic forms. If, however, we may assume as a 

 working hypothesis the existence of a great series of recumbent 

 folds* broken by subsequent block-faulting, this lithological dis- 

 tinction does not necessarily imply any difEerence in age between 

 the schists (belonging to the lower portion of the series of folded 

 sheets) and the far less altered greywackes derived from the upper 

 portion of the series. This, also, would afford an explanation of 

 the absence of pebbles of the schist, of quartz-]>ebbles derived 

 from the schists, and of micaceous sands, froan thei Mesozoic semi- 

 glomerates and greywackes adjacent to the areas oi schist, for 

 were the schists much older than the unaltered Mesozoic sedi- 

 ments, they might reasonably be expected to have yielded detritus, 

 that would be recognisable in the Mesozoic rocks as unmistakably 

 as it is in the Tertiary rocks that rest on the schist. Thus 

 Marshall's view that tha schists are altered Mesozoic, and, we may 

 add, late Palseozoic rocks seems to involve fewer difficulties than 

 any other hyi^cthesis that has been put forward. 



The explanations offered for these schists lead to interesting 

 considerations. Hutton (1900) drew attention to the gene- 

 rally low angle of dip of these rocks (thongh locally steeply 

 dipping or crumpled), and held that their metamorphism cannot 

 bo explained by lateral pressure, nor is there any evidence of 

 contact metamorphism, for plutcnic rocks are absent. He, thei'efore, 

 concluded that the change was due to regional thermal meta- 

 morphism, when the interior heat C'f the earth was greater than 

 at present. This is in marked agreement with Daly's (1917) 

 recent discussion of the efficiency of " load metamorphism," in 

 terranes " where the bedding is perfectly preserved in the sedi- 

 mentary members of the crystalline series of rocks, the dip is 

 characteristically low, even nearly horizontal over wide areas, and 

 the structure is that of a somewhat broken plateau." Even 

 where the inetamorphic rocks have been considerably dislocated, 

 as in the Erzgebirge, where there is a gradual passage from 

 gneisses through crystalline schists and thick phyllites into fossili- 

 ferous Cambrian beds, Daly holds (in opposition to certain other 

 geologists, especially Lepsius), that such metamorphism may have 

 occurred at an early period, long prior to the folding, and under 

 a comparatively small load, but with a thermal gradient much 

 steeper than existed in Palseozoic and Mesozoic sediments, even 

 though they were buried to greater depth in the crust. This last 

 point prevents the application of Daly's hypothesis to the problem 



* Since this was written, the author learns that the existence of overthrust or 

 overfolded sheets in Central Otago has been suggested bj^ Wilckens (1917). 



