president's address — SECTION c, 71 



The Problem of the Otago Schists. 



So far we have left untoixclied the difficult problem of the age 

 and origin of the Otago Schists, of which the structure-planes lie 

 inclined at remarkably low angles. These consist of quartz-mica- 

 schist, with the structures characteristic of rocks in the uppermost 

 of Grubenmann's metamorphic zones, and are intercalated with 

 bands of chlorites or hornblende, schists, which were originally 

 doleritic rocks (cf. Finlayson, 1907). From the first it has been 

 stated that these pass by decreasing metamorphism into unmeta- 

 morphosed rocks. Hutton (1875) placed them into his Wanaka 

 Series passing insensibly to north-east and south-west through the 

 semi-metamorphic Kakanui formation into the Kaikoura forma- 

 tion, the last being considered equivalent to the Te Anau-Maitai 

 Series. Later (1900) he ccnsidered them all of pre-Cambrian 

 Age. Hector (1886) suggested that they might range in age from 

 Silurian to Carboniferous. Park (1906) classed the most meta- 

 morphic schists as Cambrian, followed by semi-metamorphic Or- 

 dovician to Carboniferous rocks. That the Central Otagoi schists 

 are older than the less metamcrphcsed flanking formations, is still 

 the view of the Geological Survey (Morgan, 1914), as is illus- 

 trated by the map herewith. Marshall (1912) has urged that 

 the schists are but the altered forms oif the Permian and Mesozoic 

 sediments, and mentioned several districts where field evidence 

 seems to indicate that a gradual passage exists between the two. 

 More recently (1918), he has described the observations made in 

 the microscopical study of a series cf two dozen rock-slices from 

 specimens, taken along a 12-mile line of section from the unmeta- 

 morphosed rocks near Balclutha, northwards into the heart of the 

 schists beyond Lawrence. This collection he has permitted the 

 writer to study. The evidence therein of gradual mechanical 

 change by the crushing of the greywacke into' an exceedingly fine- 

 grained rock, and its regularly varying degree of recrystalliza- 

 tion into a schist, seems to' he very strong. The writer has not yet 

 had other opportunity for personal investigation of this problem, 

 but accepting Marshall's statement cf the field -occnrrence of these 

 rocks, the explanation which be oilers, though at first sight op- 

 posed toi onr general experience, seems to be well worthy of 

 consideration. It has been urged, by the Geological Survey, for 

 **xample, that the greywackes, which are the unaltered or initial 

 stage of the schists, are quite as like the Ordcvician ( ?) Aorere 

 greywackes, or the presumably older sediments invaded by the 

 gneissic diorites, as they are to the greywackes of the Maitai or 

 Hokonui Series, and that, therefore, an Ordovician or older age is 

 more probable for the Otago schists than a Mesozoic one. On the 

 other hand, the earlier descriptions of unconformities between the 

 schistose and unaltered rocks remain unconfirmed. It is true that 

 the two types of rock have been brought intoi juxtaposition in the 



