Vol. 50.] STRUCTURE OF CARBONIFEROUS DOUERITES AND TUFFS. 613 



2. Potluck Pseudomorphs after Olivine. 



In an ophitic olivine-dolerite from Potluck, outcrop 60, the 

 olivine is replaced by a lamellar greenish-yellow or reddish-brown 

 dichroic mineral. The greatest absorption takes place when the 

 traces of cleavage are parallel to the short axis of the polarizer. A 

 pseudomorph which shows neither cleavage-cracks nor dichroism 

 gives in convergent light coloured rings and a bisectrix at right 

 angles to the section* The plane of the optic axes is at right angles 

 to the length of the original olivine-crystal, the angle between the 

 optic axes is very small, and the double refraction negative. As a 

 rule a pseudomorph behaves as a crystallographic individual, and not 

 as an aggregate. The traces of cleavage are generally parallel to 

 the length of the crystal. In the case of two pseudomorphs, each 

 consists of two or more portions of the mineral differently orientated. 

 In one the outer portion is of a green colour without dichroism, 

 giving an optic axis outside the field and coloured rings in con- 

 vergent light. It contains two isolated kernels with cleavage 

 parallel to the length of the crystal. The other (see PI. XXIV. 

 fig. 3) consists mainly of the yellow mineral with cleavage parallel 

 to its length, and polarizes in colours of the 1st and 2nd orders. 

 It contains two isolated kernels cut perpendicular to the acute 

 bisectrix. Xo fresh olivine occurs in the slide, nor has any been 

 found in this outcrop. 



In a specimen of more weathered rock from the same locality the 

 augite and felspars are partly altered. The pseudomorphs of olivine 

 have good crystalline boundaries, and are often entirely enclosed in an 

 ophitic plate of augite. They are similar to those described in the 

 previous specimen. They are yellow when the traces of cleavage 

 are at right angles to the short axis of the polarizer, and brown 

 when these are parallel to it. One extinguishes at an angle of 37° 

 with the length of the crystal, and shows coloured rings and a 

 nearly straight axial arm. 



In a hand-specimen of the same rock glistening faces of a bronze 

 or reddish-bronze colour are seen, having the characteristic outline 

 of olivine and attaining sometimes a length of 4 millim. Examined 

 with a lens they are seen to possess cleavage, and, on the glistening 

 faces, the straight and curved cracks usual in olivine. The cleavage 

 is easy in one direction, and flakes are readily detached with a 

 knife. They often break at right angles to the cleavage-planes 

 along the cracks. When mounted, the thin flakes appear brown or 

 brownish-yellow by transmitted light. In convergent light they 

 show a biaxial figure with a small angle between the axes, and 

 negative double refraction. They are sometimes almost uniaxial. 

 When a fragment does not lie on the cleavage-plane it shows 

 dichroism, the greatest absorption taking place when the short axis 

 of the polarizer is parallel to the traces of cleavage. 



Similar pseudomorphs are found in outcrop 42, near Upper- 

 wood on Masson Hill. The felspars are slightly turbid, and no 

 fresh augite or olivine is present. A six-sided section is bounded 



2u2 



