Park.— 0/i the Geology of North Head, Waikouaiti. 419 



tion not readily dispelled until the face of the rocky escarp- 

 ment above is carefully examined. Proceeding, however, 

 along the sea-rim of the talus from the end of the sandy 

 beach the masses of rock forming the sea-wall are found to 

 lie tumbled in all directions. These circumstances may be 

 explained on the reasonable assumption that, the line of slid- 

 ing being necessarily more or less parallel to the face of the 

 escarpment, the descent of the moving masses on the slippery 

 surface of the underlying blue clays next the inlet would be 

 uniform and gradual, whereas at the north-east end of the 

 escarpment, where the supporting clays would be subject to 

 .undercutting by the sea, the overhanging sandstone would be 

 liable to break away in large blocks whenever the unsupported 

 weisfht of the mass became greater than the transverse 

 strength of the material. 



Mount Cronin, when viewed from the railway-line on the 

 south side of Waikouaiti Inlet, is seen to be dome shaped, 

 and to surmount the surrounding country by several hundred 

 feet. 



General Geological Structure. 



A general view of Mount Cronin shows that it is composed 

 of a series of marine sandy beds now consolidated into soft 

 sandstones. These sandstones rest confoimably on marme 

 blue clay, and are overlain unconformably by gravel and silt 

 beds, which in their turn are crowned by a flow of basalt. 



The basalt occupies the summit of the hill, and there can 

 be little doubt that this commanding eminence owes its pre- 

 sent existence to this hard cap, which has protected the 

 underlying loosely compacted gravels and silts from destruc- 

 tion by the ever-active agencies of subaerial denudation. 



Omitting the sand-dunes, slope deposits, and other recent 

 accumulations, we have thus in this interesting section three 

 distinct formations — one marine, one fresh- water or terres- 

 trial, and one volcanic. The peculiar association of these 

 formations and their relations to each other throw a strong 

 light on the geological history of Dunedin, and enables us to 

 read aright sections heretofore involved in obscurity. 



Classification of Eock-formations. 



To facilitate description the rock-formations occurring at 

 Mount Cronin may be classified as follows : — 



Sand-dunes and talus deposits Eecent. 



Mount Cronin basalt ... New^er Pliocene. 



Waikouaiti leaf beds ... Older Pliocene. 



Waikouaiti sandstone ■■■) r\ 



TP ■ -c 1 Oamaru series. 



i oraminiferous clays . . , i 



