614 MR. H. H. ARNOLD-BEMROSE OST THE MICROSCOPICAL [Nov. 1894, 



by the faces (010) (110). The angle (110) A (110) is 130°. It is 

 subtended by the traces of cleavage, which are therefore parallel to 

 100, the macropinacoid of the original olivine. In another section 

 the cleavage-traces subtend the angle 135°. Yet another section 

 gives an angle of 77° between the dome faces. This is the angle 

 for olivine in sections parallel to the brachypinacoid (010). The 

 cleavage-traces are parallel to the length of the crystal and therefore 

 to the C axis. Combining these results, it follows that the planes 

 of cleavage are parallel to the macropinacoid, as in the case of 

 iddingsite described by Lawson. A pseudomorph is often composed 

 of several minerals which are arranged in zones parallel to the 

 outline of the crystal. We have an outer zone of the mica-like 

 mineral, with an inner portion of calcite and oxide of iron ; or an 

 outer zone of iron oxide, followed by a thick one of the mica-like 

 mineral, and this again by a thin zone of iron oxide and a nucleus 

 of calcite. All traces of the original oli vine-cracks are lost, but the 

 pseudomorph sometimes contains wide cracks filled with calcite. In 

 some cases the olivine is replaced by iron oxide, with or without an 

 outer border of chlorite or serpentine — pale and slightly dichroic, 

 the fibres being irregularly arranged. 



A specimen from New Bridge, near Ashford, outcrop 30 r 

 contains pseudomorphs similar to those from Potluck, except that 

 most of the original cracks in the olivine are filled with iron oxide. 

 In a more highly altered specimen (see PL XXIV. fig. 4) the pseudo- 

 morphs are dark green for rays vibrating parallel to the short axis 

 of the polarizer, and light green or faint yellow for rays vibrating at 

 right angles. The double refraction is strong, and the polarization- 

 colours are similar to those of biotite. The cracks are filled with 

 iron oxide, which has in some cases replaced more or less of the 

 olivine nucleus. Cleavage-flakes taken from a hand-specimen behave 

 much as those from Potluck. In several sections the cleavage-cracks 

 subtend angles varying from 134° to 126° (110 A 110), so that the 

 traces of cleavage are parallel or nearly parallel to the macropinacoid. 

 The whole pseudomorph has not always the same optical orientation 

 throughout. In a section which gives an acute angle of 71° between 

 the traces of the dome faces, the greater portion gives an acute 

 bisectrix perpendicular to the section, and the plane of the optic 

 axis is parallel to the trace of one of the dome faces. It contains 

 three patches with dichroism and cleavage parallel to the length of 

 the crystal. 



3. Peak Forest Pseudomorphs after Olivine. 



In Dam Dale, close to the village of Peak Forest, there is a good 

 outcrop (no. 4) of ophitic olivine- dolerite. Seven thin sections have 

 been examined. In the most weathered ones the augite is little 

 altered, but the felspars are turbid. Pseudomorphs of olivine 

 similar to those at Potluck are found. Instead of behaving as a 

 crystallographic individual, the replacement-product sometimes 

 consists of fibres irregularly arranged, in other cases part of the 



