Speight. — The Geology of Banks Peninsula. 381 



Since nearly all the dykes except those at the head of the harbour, which 

 are almost entirety trachytic, have a radial orientation, and therefore do 

 not intersect, no satisfactory conclusion can be come to as to the relative 

 age of the trachytic and basic varieties- — -if, indeed, they do belong to 

 different periods of intrusion. The only intersections that I am aware of 

 are those of trachytic dykes, and they do not furnish any sound grounds 

 for differentiating between the earlier and later trachytes on the score of 

 composition. 



(b.) Akaroa. 



Basic Flows, Trachyte Dykes, and Syenite. — The general features of the 

 Akaroa volcano are analogous to those of Lyttelton. It has been con- 

 structed in the same way of alternating flows of basic rock and fragmentary 

 material, but fine- and coarse-grained basalts of normal habit form the 

 great majority of the flows, basalts of andesitic habit being rare. Some 

 of the flows, too, seem to be specially subject to weathering agents, so that 

 there is a thicker coverino; of fertile soil than in the case of Lyttelton ; but 

 hard resistant basalts are very common, and seem to determine by their 

 presence the marked shelves of flat ground which are evident on the sides 

 of the harbour, and also the summits of the higher peaks, elevations such 

 as Mount Bossu, Saddle Hill, French Peak, and Brasenose being composed 

 of flows of lava showing little signs of decay, their slopes facing the harbour 

 being precipitous and bold, while their external slopes are comparatively 

 gentle and accordant with the angle of the lava-flows. Judging from the 

 present spread of the base and the inclination of the lava-streams, the 

 volume of the Akaroa cone far exceeded that of Lyttelton. 



Like this it had also a series of radiating dykes, which are generally 

 trachytic in character, but neither their number nor the volume of the 

 output appears to have approached those of Lyttelton. This may, how- 

 ever, be deceptive, since the maturity of erosion and depth of dissection 

 are not so marked in the case of Akaroa. and therefore dykes have not 

 been so extensively exposed. In the neighbourhood of Onawe they depart 

 from the radiating arrangement and form on the shore platform a criss- 

 cross pattern, with no regular orientation, except perhaps that more run 

 in a N.E.-S.W. direction than in any other ; but this conclusion may be 

 erroneous. 



A most interesting occurrence in the Akaroa area is a coarse-grained 

 plutonic rock- — a hornblende syenite, of peculiar type — which forms the 

 rounded hill terminating the pear-shaped peninsula of Onawe. The con- 

 tacts between this and the surrounding rocks are everywhere obscured by 

 soil and debris even down to low-water mark, so that its field relations 

 cannot be worked out satisfactorily. It apparently represents a volcanic 

 rock which has consolidated at a deeper level and has been exposed owing 

 to erosion, and is no doubt connected genetically with the trachyte dykes 

 of the area, which form an intersecting network at the base of the peninsula 

 referred to previously. But one at least of these dykes on the western 

 side intersects the syenite as well as the later basic volcanics, so that the 

 syenite may be a fragment of an older land-mass, and not directly connected 

 with the later volcanic period. Just where this trachyte dyke intersects it 

 there is a small exposure of an extremely basic facies of the rock containing 

 numerous grains of magnetite, and the sand on the beach close to it is largely 

 composed of this material, as well as hornblende and feldspar. The form 

 of the end of the peninsula is one specially characteristic of granite rocks, 

 and the material in position is deeply weathered. 



