180 Transactions. 



Office, and it has been used in the construction of others. Blocks 

 of this stone occur in the archway over the entrance to Canter- 

 bury College. As a building-stone it is very easy to work, and 

 stands the weather extremely well ; but its appearance is some- 

 what spoiled by the presence of fragments of the country rock, 

 which are irregularly distributed through it. 



Macroscopic Appearance. 



The rock is of a light greenish-grey colour, with phenocrysts 

 of feldspar visible in a rather porous groundmass. A number of 

 black specks also are to be seen, and these are either the soda 

 amphibole or aggregates of iron-ore derived from it. No other 

 porphyritic crystals are visible. The included andesitic frag- 

 ments are of all sizes, up to 10 cm. in length. 



Specific Gravity. 



The specific gravity determined immediately after immer- 

 sion in water was 2-35 ; on leaving the rock to soak for twenty- 

 four hours it was 2-48 ; and determined by a specific-gravity 

 bottle it was 2-57. These figures afford some idea of the vesi- 

 cular character of the rock. 



Chemical Analysis. 



A chemical analysis of the rock was made in the chemical 

 laboratory, Canterbury College, by several students, under the 

 direction of Dr. W. P. Evans, who has kindly furnished me with 

 the following result : — 



Per Cent. 

 Si0 2 70-04 



A1,0 3 



FeA 



FeO 



CaO 



MgO 



Na 2 



K 2 



H 2 



15-40 



4-65 



Slight trace onlv. 

 0-55 

 4-35 

 4-65 

 0-57 



100-21 



The following points with regard to this result are specially 

 noticeable : The high percentage of Si0 2 (70-04), the low per- 

 centages of CaO and MgO, and the moderately high percentage 

 of alkalies for a rock of its character. These peculiarities are 

 explained by the microscopical examination, and will be deall 

 with more fully subsequently. 



