Wall. — Distribution of Senecio saxifragoides Hook. f. 



199 



mass of the peninsula was built up in mid-Tertiary times by flows of basalt 

 and fragmentary material of similar lithological character, poured out from 

 two vents situated somewhere near the centres of Akaroa and Lyttelton 

 Harbours. A third focus of activity lay near Mount Herbert (3,012 ft.), 

 but it was of less importance, although it was responsible for the formation 

 of the highest peak in the area. The high cones thus formed were subject 

 to paroxysmal explosions of moderate intensity, and their surface was modi- 

 fied by the establishment on their outer slopes of a well-developed system 

 of radiating valleys. Volcanic action ceased in all probability long before 



SMt Pleasant 



Sugar Loaf o LYTTELTON 



Cass Peak o 



s •. 



p 



Cooper's Knob o 



5- 



^ M 



Castle 

 Rock 



Ml. Herbert 



' , Mt. Sinclair 



• L .°.l 



° ° ° o 



c Mt Filigerald ° » 



Rocky Peak ° 



Map of Banks Peninsula and Port Hills, showing distribution of the two speciea of 

 Senecio. L, Senecio lagopus ; S, Senecio saxifragoides; A, rhyolite escarpment 

 where S. lagopus occurs ; B, rhyolite escarpment where neither species occurs. 



the end of the Tertiary era. After the stream-system had reached a mature 

 stage the land sank, and the sea entered the floors of the enlarged craters 

 and extended a considerable distance up the lower reaches of the valleys, 

 and these now form marked indentations of the coast-line. Owing to the 

 prolonged weathering the land is now covered with a rich and fertile soil, 

 and steep rock-faces occur only on the coast and at higher levels, where 

 the more resistant basalts form at times precipitous cliffs — the characteristic 

 habitat of the senecios under consideration. 



The following are the most important geological considerations affect- 

 ing the distribution and ecological conditions of plants established in the 

 locality : — 



(1.) The isolation of the region from neighbouring mountain areas since 

 it was first formed. 



