374 MB. A. M. FLNLAYSON ON THE NEPHBITE [Aug. I9O9, 



from. Prof. Iddings, again, suspects that some small red-brown 

 isotropic grains with an opaque border like leucoxene, in a New 

 Zealand specimen examined by him, may be perovskite [12, p. 94, 

 specimen 286]. Similar grains were present in most of the slides 

 examined in the course of the work ; but they showed no appearance 

 of a leucoxene border, and they have been referred provisionally 

 to spinels. The accessories are in all cases so small that an exact 

 determination is difficult. 



The following three special types throw light on the origin of the 

 nephrite : — 



(1) The grey fibrous amphibole (PI. XV, fig. 5) which has been 

 described as occurring with the serpentine of the Dun Mountain, 

 and due to contact-action, is essentially a variety of nephrite, 

 resulting from dehydration of and addition of lime to the serpentine, 

 or simply from the addition of lime to the original anhydrous 

 magnesium silicate. 



(2) Certain of the nephrites show, under the microscope, grains 

 of more or less uralitized pyroxene (PL XVI, fig. 5). The 

 pyroxenes, when observed, are very pale or colourless, with a 

 rounded outline, and an extinction-angle (when the mineral is 

 sufficiently unaltered for measurement) of 35° to 40°. The edges 

 of the grains are frayed out into colourless fibrous amphibole 

 (nephrite), which mingles with the surrounding aggregate ; and the 

 main mass of the individuals is partly or wholly transformed into a 

 patchy aggregate of fine, interlacing, tufted groups of nephrite- 

 fibres, which extinguish roughly as a whole, thus enabling the form 

 and boundaries of the original pyroxene crystal to be made out. 



The same mode of transformation of augite into nephrite was 

 observed and described by Dr. Dieseldorff in specimens fromD'Crville 

 Island [5, pp. 340 et seqq.~]. I have found it only rarely in the 

 West Coast specimens, the uralitization being generally too complete 

 to enable the process to be followed. 



(3) A specimen may here be described which illustrates still 

 another mode of derivation of nephrite, and one, so far as I know, 

 hitherto not observed in nephrites from any locality. The rock in 

 question has not been recorded in situ, and was secured, apparently 

 without a definite locality-label, from the New Zealand & South 

 Seas' Exhibition held in Dunedin in 1890. It is believed to have 

 come from the region of peridotites described by Ulrich in North- 

 Western Otago. For specimens of it I am indebted to Prof. Marshall. 

 It is dense and tough, fine in grain, greyish black in colour, and 

 has a specific gravity of 3-012. 



Under the microscope the rock shows the direct transformation of 

 olivine into fibrous amphibole (PI. XVI, fig. 6). It consists of a 

 mosaic of rounded olivine grains, pierced in all directions by a crossing 

 and confused network of very fine amphibole fibres. The residual 



