Speight. — On an Occurrence of Olivine-ancleslte. 367 



Summary. — The origin of the earthquake felt around Cook 

 Strait on the 4th December, 1891, was beneath an area most 

 probably including E' and K', at a depth possibly somewhat 

 less than 10 miles ; the velocity of propagation was slow- 

 about 708ft. per second ; the time, at the origin, of the chief 

 shock was 4k. 33min. a.m., nearly ; the maximum recorded 

 intensity, vii. on the Eossi-Forel scale.* 



Art. LI. — On an Olivine-andesite of Banks Peninsula. 



By E. Speight, M.A., B.Sc. 



[Read before the Philosophical Institute of Canterbury, 2nd June, 2892.] 



Plate XLVII. 



Varieties of this rock are found in several places on Banks 

 Peninsula, either as dykes or as sheets of lava (vide " Pro- 

 ceedings of the Eoyal Society of New South Wales, 1889," 

 page 142), but the particular rock under question occurs on 

 the Port Hills, about three miles from Christchurch. It forms 

 the main part of one of the spurs of the hills which stretch 

 out into the. plain in a north-westerly direction. As there 

 are no clear-cut faces, the only place where a good sec- 

 tion can be obtained is at a small quarry on the west side of 

 the spur, where stone has been obtained for building parts of 

 the Canterbury Museum and Canterbury College. As this 

 quarry is of small extent it does not give much evidence to 

 determine from a section whether the rock is a dyke or a lava- 

 sheet. There appears nowhere a parting which might mark 

 the wall of a dyke, but there is positive evidence which is 

 almost conclusive that it is a lava-sheet. 



1. The size of the spur, which is several hundred yards 

 across, capped all round by this rock, renders it probable that 

 it is not a dyke. 



2. The vesicular nature of the rock shows it was a sub- 

 aerial flow, since, if it had consolidated between the walls of a 

 fissure, vesicles would be absent. 



3. The rudely prismatic manner in which the rocks are 

 jointed, with the axes of the prisms vertical and not horizontal, 

 is additional proof that it is not a dyke. 



Included in the rock are numerous rounded fragments of a 



* The Rossi-Forel scale was adopted as a convenient standard of 

 intensity by the Seismological Committee of the Australasian Association 

 at the Hobart meeting, January, 1892. 



