Speight. — The Geology of Banks Peninsula. 



379 



occasionally found sandwiched in between trachytes, so that a definite pro- 

 nouncement must be taken with considerable reserve. 



Dykes of both species appear at the very crest of the crater-ring, but it 

 is remarkable that the trachytes, though apparently soft and non-resistant, 

 stand out freely as massive walls sometimes as much as 70 ft. above the 

 surrounding country. As might be expected, the basic dykes are more 

 common on the outskirts of the volcano, while the massive and more viscid 

 trachytes are as a rule short in length and somewhat lenticular in shape. 

 There are. however, exceptions to this general rule. 



On the south side of Quail Island the trachyte dykes are unusually 

 numerous, and they are apparently the only ones occurring. The foreshore 

 from the outer wharf to the extreme south -westerly point of the island is 

 intersected by them, nearly sixty occurring in the space of a mile in length. 



Sctr/p e/,J///Vs 



/ 



/ 



Fig. 3. — Map showing tin- system of radiating and divergent dykes belonging 



to Lvttelton Harbour. 



They vary in size from mere ribbons up to masses 12 ft. across. In some 

 cases injection appears to have taken place twice up the same fissure, or 

 the original dyke appears to have been disrupted and injection taken place 

 up the fissure so formed. Most of the dykes are vertical or nearly so, but 

 occasionally they are flat like sills and are injected along the planes of flow 

 of the rhyolite. Friction breccias are also sometimes in evidence. The 

 orientation of the dykes in this locality varies through a right angle, so that 

 looking south from the shore they lie in the quadrant extending from south- 

 south-east to west-south-west, with one or two having a more east-and-west 

 trend. The great majority, however, are directed between south and south- 

 west. Owing to the variation, they occasionally cross, but there is no 

 regularity in the direction of those crossing later from which a conclusion 

 may be drawn as to the existence of more than one focus of eruption,- the 

 variation in all probability being due to proximity to the single centre of 



