Marshall. — On Trldymite-TrachijU of Lyttclton. 387 



Plate XLVI. 



Fig. iii. is a section in which tridymite occurs to a greater extent than 

 usual, a, b, and c are grains of this mineral ; e and d are untwinned 

 sanidine ; / is a grain of magnetite. In all of these sections there is 

 no attempt made to give the exact appearance of the ground-mass, 

 since beneath such a low power it merely presents a cloudy appear- 

 ance. 



Fig. iv. shows a large crystal of plagioclase surrounded by a ring of sani- 

 dine, with a small layer of differently-oriented feldspar between, a is 

 the sanidine, b the intermediate layer of plagioclase, and c the central 

 core of plagioclase ; a is extinguished in polarised light when the 

 trace of the cliuopinacoid makes an angle of 26° with the cross-wires ; 

 6 is extinguished when the angle is 11'5°, and c when the angle is 

 33°. The colouring shown in the drawing is seen when the trace of 

 the cliuopinacoid makes an angle of 64° with the cross-wires. 



Plate XLVII. 



Fig. V. a is a crystal of sanidine which was described when treating of 

 the mineral generally ; 6 is a vesicle ; c is another sanidine crystal. 



Fig. vi. This also was described previously, a is the outer ring of sanidine ; 

 b is the central portion of plagioclase ; c is a mass of iron-ore ; e is 

 tridymite ; /, air-space, or vesicle ; while d is sanidine that has 

 apparently intergrown with the larger crystal, but has independent 

 orientation. 



Plate XLVIII. 



Fig. vii. is a crystal of tridymite as seen under a magnifying-power of 

 70 diameters. The outlines of the different plates are quite notice- 

 able with this power. 



Fig. viii. is another tridymite grain in which an attempt is made to show 

 the radial structure that is often so noticeable with polarised light. 



Fig. ix. is the supposed Baveno twin, also magnified 70 diameters. Two 

 opposite quarters of this crystal extinguish when the line joining 

 a and b makes an angle of 11° with the cross-wires. The other 

 two extinguish when the angle is 18°. When the angle is increased 

 to 23° they are indistinguishable, and remain so until the angle is 

 79°. 



Fig. X. was previously mentioned as a combination of albite and pericline 

 twinning, a a is the surrounding sanidine ; b b are lamellae, appa- 

 rently of albite twins on a broad pericline face ; c is twinned on the 

 albite and d on the pericline law ; e and / are also pericline lamellae, 

 twinned according to the albite law ; h is another pericline plate, 

 which in certain positions shows secondary twinning ; n appears to 

 be an albite lamella which shows no secondary twinning ; m m are 

 magnetite grains. The appearance shown in the figure is presented 

 when the lines of albite twinning make an angle of 27° with the cross- 

 wires. 



Fig. xi. is a compound grain of sanidine and plagioclase. a a a, &c., are 

 sanidine grains, all with different orientation ; b b are plagioclase ; 

 and c c are grains of magnetite. This kind of combination is very 

 common in the rock. The last two figures are magnified 30 dia- 

 meters. 



All the crystalline outlines were drawn with the camera lucida, and, 

 when shaded, polarised light was used. 



