

Vol. 50.] STRUCTURE OP CARBONIFEROUS DOLERITES AND TUFFS. 615 



pseudomorph consists of these fibres, and part of the lamellar 

 mineral. Cleavage-flakes behave like those from Potluck. 



There is also a second kind of pseudomorph which is found only 

 in the less altered rock. Several of the larger olivines are entirely 

 replaced by a pale yellow mineral, with traces of cleavage which are 

 parallel to the long axis of the crystal, but which do not always 

 run through its whole length. The cracks are filled with and 

 bordered by a clear yellow substance («), and this in turn is bordered 

 by a pale yellow one (6). Both are bounded by lines parallel to the 

 crack, except where two cracks meet, when b fills up the triangular 

 space between them ; or where two parallel cracks are near to- 

 gether, when 6 tills the space between them. Both are dichroic, 

 a becoming a darker yellow when the short axis of the polarizer is 

 parallel to the length of the crystal, and o being yellow when 

 the short axis of the polarizer is perpendicular to the length of the 

 crystal, and a ' solid '-looking bluish-green when it is parallel. Both 

 a and b extinguish together over the whole crystal, parallel to the 

 long axis of the section. In some pseudomorphs the mineral is 

 non-dichroic, and extinguishes at an angle of 40° or 46° with the 

 length of the olivine-crystal. 



An olivine-crystal, more than half of which is unaltered, has 

 the plane of the optic axes at right angles to its length. The 

 cracks are bordered by a, which is non-dichroic and yellow, and 

 a in turn is bordered by b with slight dichroism. Both these 

 substances become extinct parallel to the length of the crystal, no 

 matter in! what direction the cracks run. The green substance (b) 

 is followed by an irregular zone of partly altered olivine, which sends 

 out shoots or fangs into the fresh olivine-nuclei. Sometimes the 

 green substance is non-dichroic, and has no traces of cleavage. 

 Examined under convergent light, with a T V-inch immersion-lens, 

 both a and b show in such cases the emergence of a negative 

 bisectrix, the angle between the optic axes being very small. 



Cleavage-flakes taken from the outer and inner portions of a hand- 

 specimen were examined. When non-dichroic, they give in con- 

 vergent light an almost uniaxial figure with coloured rings havinq- 

 negative double refraction, the acute bisectrix being perpendicular to 

 the plane of cleavage. In some fragments both the yellow and 

 green kinds are seen, showing that they do not easily become 

 detached one from the other. The cleavage is seldom very perfect, 

 and the fracture is often fibrous. 



The cleavage (when it occurs) is often parallel to the macropinacoid 

 of the olivine, as in the case of the first kind of pseudomorph or that 

 found at Potluck and other places above described. Olivine-nuclei 

 showing the emergence of a positive bisectrix at right angles to the 

 section, and therefore cut parallel to the macropinacoid, polarize in 

 green and greenish-yellow of the first order. In such cases the 

 pseudomorphous parts are not dichroic, and show no traces of 

 cleavage. Nuclei cut parallel to the negative bisectrix and therefore 

 parallel to the brachypinacoid, polarize in yellow and blue of the 

 first order. The pseudomorphous part is dichroic, and polarizes in 



