626 PROF. G. H. F. ULRICH ON THE 



been able to test the correctness of my supposition by microscopic 

 examination. In thin sections both olivine and enstatite become 

 nearly or quite colourless and transparent, the former in parts with a 

 faint greenish, the latter more generally with a feeble brownish-yellow 

 tint ; and they are on the whole rather free from inclusions, chromite 

 and picotite excepted. In PL XXIY., figs. 1, 3, 5, grains of greenish 

 olivine are indicated by the letter o. I suspected these at first to be 

 the green mineral before mentioned, but their rough surfaces and high 

 brilliant polarization-colours unmistakably proved them lo be olivine. 

 Between crossed W^icols the enstatite, determined by always extin- 

 guishing parallel to the main cleavage-cracks, shows in some grains 

 thioughout, in others in parts, a closely and sometimes slightly-curved 

 fibrous structure, which may indicate its alteration into bastite*. 

 On looking straight through a mounted section towards the light 

 with the naked eye, enstatite and olivine are often indistinguishable 

 from each other, if both are clear and colourless ; but on looking 

 obliquely down upon the section, whilst turning it in different direc- 

 tions between the fingers, the grains of enstatite are recognizable 

 by a peculiar brightening-up like opalescence, caused by internal 

 reflections. Chromite and picotite, in opaque black and brownish 

 translucent crystals and irregular grains, occur in moderate number 

 in every section. The dark-green translucent serpentine from the 

 Eed Hill becomes, in thin sections, perfectly transparent and nearly 

 colourless, with scattered patches of a slightly cloudy aspect ; and it 

 encloses numerous black and brown opaque grains of iron-ores, some 

 of which on microscopic examination by reflected light generally prove 

 to be pyrite. Between crossed Nicols the clear substance appears as 

 broken up into a confused intermixture of small granules and narrow, 

 longer and shorter fibrous bodies, most of which (the fibrous bodies 

 invariably) are anisotropic, by polarizing from bright white or yel- 

 lowish through light and dark shades of greyish-blue to black, the 

 extinction of the latter being in the direction of the fibres. The rest 

 of the granules that are dark remain so on complete rotation of the 

 stage, thus proving to be isotropic. Irregularly outlined portions 

 in the sections, showing more minute granulation, no fibrous bodies, 

 and polarizing in parallel bands in diiferent shades of blue and grey, 

 are doubtless indicative of altered enstatite, and conform to the 

 cloudy patches before adverted to. 



It requires now finally to be mentioned concerning the Eed-Hill 

 rocks, that, as far as Messrs. Henderson and Batement could observe, 

 there exists no defined boundary between the unaltered peridotite 

 and the serpentine. The two rocks seemed to be quite irregularly 

 intermixed. Rock-masses which at a distance they took to be 

 peridotite proved on examination to be serpentine and vice versa, the 

 recognition being always easy on near approach, even without break- 

 ing the rock, by the rough surface of the peridotite and the smooth sur- 

 face of the serpentine. They came in their traverses through the 

 district at several places, pretty far apart, across the boundary of the 

 schist and peridotite rocks (including serpentine); and, judging from 

 * J. J. H. Teall's < British Petrography,' p. 88. 



