388 PROF. P. MARSHALL ON THE [Aug. 1906,. 



deep into the rocks, and thus the work is rendered less difficult ; 

 though the want of connexion between the sections is especially 

 confusing in such a district as this, where changes so sudden and 

 complete occur in places where the margin of one lava-sheet is 

 reached, or a sudden uneven n ess of the surface over which it flowed 

 causes it to dip unexpectedly beneath a totally-different rock. For 

 these and other reasons, of which one is the comparatively-large 

 extent of the district, the present paper must be regarded as pre- 

 liminary, although it is thought that the observations already made 

 enable the general structure of the district to be correctly de- 

 scribed, despite the fact that the details of nearly all portions are 

 quite inadequately known. 



Sixteen distinct rocks were described by the late Prof. Ulrich. 1 

 All the rocks that he examined were classed as phonolites and 

 andesites, though he remarked that some varied toward tephrites 

 and basanites. Many of these I have classed as trachydolerites. 



Hock-specimens have been collected in fair number from portions 

 of the district, and from these over a thousand microscopic sections 

 have been prepared. The following classes of rocks have been 

 recognized : — ■ 



I. Metamorphic : 

 Mica-schist. 



II. Sedimentary : 



1. Kainozoic sandstones and limestones. 



2. Conglomerate. 



3. Recent alluvium. 



III. Igneous 



1. Hornblende-foyaite. 



2. Augite-diorite. 



3. Tinguaite. 



4. Ulrichite. 



5. Hypabyssal trachydolerite. 



6. Camptonite. 



7. Teschenite. 



8. Trachyte. 



9. Kaiwekite. 



10. Tracbytoid phonolite. 



11. Nephelinitoid phonolite. 



12. Leucitophyre. 



13. Trachydolerite. 



14. Andesite. 



15. Nepheline-basanite. 



-in f Polerite, Mount- Charles type. 

 ' \ Dolerite, Papanui type. 



17. Basalt. 



18. Melilite-basanite. 



19. Port-Chalmers breccia. 



Metamorphic rocks. — The age of the mica-schist is not 

 definitely known. By the late Capt. Hutton it is, on general grounds, 

 referred to the pre-Carnbrian Era. By Sir James Hector — whose 

 opinion is chiefly based on the petrographic similarity of the schist 

 to certain rocks the position of which is definitely known in the 

 Nelson province — the mica-schist is referred to the Silurian Period. 



Whatever its true age, the complete metamorphic nature of the 

 rock without steep or complicated folds, and its entire uncon- 

 formity to all other formations of the district, prove that it has been 

 very deeply buried. Its emergence at the surface is the result of 



1 Eep. Austral. Assoc. Adv. Sci. vol. iii (1891) pp. 127-50. 



