Marshall. — On Tridymite-Trachyte of Lyttelton. 377 



it appears between crossed uicols when the longer side of the 

 crystal is inclined at an angle of 33° with the cross-wires. It 

 would appear that h, e, cl are broad lamellae of pericline twins, 

 n being a tongue of feldspar twinned on the albite law, the 

 lamellae appearing on rotating the stage. The lamellae li, e, d 

 are themselves striated with albite lamellae, which also appear 

 at e, c. The lamella d, on the other hand, has subsidiary 

 striation due to twinning on the pericline law. c d, e f, all 

 extinguish together, as do n h ; m m are grains of magnetite. 

 The whole is surrounded with a mantle of sanidine, a a. 



Magnetite is present in every section, but varies consider- 

 ably in abundance. The grains exhibit great variation in 

 boundary, as the usual form of the mineral is the rhombic 

 dodecahedron and octahedron. Some grains show a fairly 

 regular hexagonal boundary, and it is possible that ilmenite, 

 which crystallizes in the hexagonal system, is present. 



Augite is of doubtful occurrence, for though between thirty 

 and forty sections have been cut there is no single occurrence 

 of this mineral which cannot be questioned. One section 

 shows an opaque crystal with octagonal outline, which is pro- 

 bably a section of augite in which both prisms as well as pina- 

 coids are developed. As, however, almost the whole of the 

 crystal has been changed into iron-oxide, its optical properties 

 ■cannot be investigated. 



Apatite is present in numerous prismatic needles which 

 pierce the feldspar and ground-mass, and was therefore the 

 first mineral to crystallize out of the magma. 



Zircon seems to be represented in one slide where there are 

 two short prismatic crystals terminated by obtuse pyramids, 

 and possessing straight extinction as well as strong double re- 

 fraction, indicated by brilliant colouring between crossed nicols. 

 It would, however, be rash to assert from these isolated ex- 

 amples that zircon occurs in the rock, especially as it has been 

 found in no other slides, including those prepared by Professor 

 Ulrich. 



The ground-mass of the rock is composed of feldspathic 

 microlites, tridymite, and magnetite, and in rare instances there 

 are globules of a greenish glass. Beneath the quarter-inch 

 objective it cannot be determined to what species of feldspar 

 these microlites belong, but, since sanidine has in the porphy- 

 ritic crystals evidently crystallized subsequently to the plagio- 

 clase, we have a certam amount of right to infer that the 

 microlites consist chiefly of sanidine. Irregularly-bounded 

 tablets of tridymite will be observed in very thin sections to 

 occupy a large portion of the ground-mass, but in thicker sec- 

 tions they are frequently over- or underlaid by microlites. It 

 will be shown afterwards that a large proportion of the base 

 must consist of tridymite. 



