386 Transactions.— Geology. 



Although it is tempting to generahse as above, it must be 

 carefully borne in mind that only three lavas have been found 

 extruded from this vent, and there is little doubt that a more 

 detailed examination would reveal the existence of other lavas, 

 while there may also have been several previous eruptions, 

 the lavas being covered by later accumulations. 



It would therefore be safe to say, of all the rocks of this 

 system so far examined, that the order of succession seems to 

 be in accordance with that demanded by Durocher's theory. 



Summary. — The tridymite - bearing rock mentioned in 

 Haast's " Geology of Westland and Canterbury " was erupted 

 from a dyke formed during the paroxysmal convulsions of the 

 central crater after its actual activity had ceased. It is not 

 interstratified with the other volcanic rocks of the Lyttelton 

 system. After the lava-stream had been formed fissures were 

 torn open in it by continued paroxysms of the central volcano, 

 and magma was forced into them, thus forming dykes through 

 the consolidated lava. 



The absolute age of the rock cannot be determined, but its 

 age relatively to the products of the large crater can be easily 

 ascertained. Investigation of its mineralogical and chemical 

 constitution shows that the rock should be classed with the 

 trachytes, its special characteristics being denoted by the name 

 andesine-tridymite-trachyte. The rocks that have been ex- 

 truded from the vent appear to be in the order of succession 

 demanded by Durocher's theory. 



EXPLANATION OF PLATES XLIV.-XLVIII. 

 Plate XLIV. 

 Fig. 1. Chart of part of Lyttelton Harbour. 



Fig. 2. Enlargement of i^art marked A in fig. 1 : OE, dyke at the top of 

 the hill ; R, Lyttelton-Sumner Road ; K, first outcrop. 



Plate XLV. 



Pig. i. is a section showing the general structure of the rock. The 

 magnifying power is 30 diameters, a o^ are crystals of tridjmite, 

 the characteristic structure being shown more clearly than it actually 

 appears under the microscope ; b is sanidine, with almost rectangular 

 boundaries, to which the cleavage is parallel ; c is an irregular segre- 

 gation of oystals of sanidine, the boimdaries between the parts 

 possessing different orientation being indicated by lines — none of 

 the grains show any striation, and the boundaries are irregular ; cl and 

 e arc grains of iron-ore, probably magnetite, but possibly ilmenite ; 

 / is a vein of hpematite or limonite that has evidently been formed 

 during the wcatherirrg of the rock. 



Fig. ii. is also a general section magnified the same number of diameters 

 as fig. i. a and b are grains of tridymite ; c is a feldspar crystal con- 

 sisting of an irregular intergrowth of two crystals, sanidine and 

 plagioclase, the former in both cases surrounding the latter ; the 

 large crystal is an irregular Carlsbad twin ; / is a compound crystal 

 of sanidine similar to c in fig. i ; e, d, and h are other crystals of 

 sanidine ; h is a large grain of magnetite. 



