Professor Bonney — On Limburgite from Sashach. 415 



groundmass is rather variable, being in some parts fairly clear, in 

 others a sort of micro-ophite or micropegmatite, composed of ferrite 

 (or some rust-brown mineral) and felspar. The colourless belonites, 

 mentioned already, are rather abundant. Evidently they con- 

 solidated at a very early stage, for they are often numerous in the 

 felspar ; they have a higher refractive index than the felspar, and 

 the general aspect of fibrolite, but rather low polarization tints and 

 oblique extinction. This prevents me from referring them to that 

 mineral, which otherwise they resemble. 



(5) Comes from the bottom of the pit (about the middle), 

 representing the lowest rock exposed. This apparently differs much 

 from the others. Its colour is greenish-black, slightly mottled in 

 places with a paler green, so that the augite crystals are less 

 conspicuous. Cavities are vexy few, small, and filled with a pale 

 grey -green mineral. Had I been asked to name the specimen 

 without knowing whence it had come, I should have replied, 

 "probably a picrite." But the microscope shows the differences 

 to be only varietal, the colour being due to the absence of the rust- 

 brown iron oxides so common in the others, and the substitution of 

 a green alteration product in the olivines,^ as is commonly seen in 

 dark-green serpentines. The groundmass of this specimen is also 

 a little variable, some parts exhibiting the intercrystallization of 

 a basic material and felspar described above, while the latter mineral 

 more commonly forms a clear groundmass of fair-sized crystals, in 

 which the others are scattered ; it is, in fact, still more nearly 

 a normal holocrystalline rock than any of the preceding specimens. 



The specimen which I collected in 1895 is megascopically very 

 like Nos. 2, 3, and 4, but under the microscope presents a close 

 resemblance to Nos. 3 and 4, as well as (allowing for the absence of 

 the green mineral) to No. 5, with one or two varietal differences. 

 The elongated little prisms of brown augite are more numerous than 

 in the latter three ; the micropegmatitic structure is much rarer, 

 for it occurs only as an outgrowth from two or three of the large 

 augites ; the minerals, large and small (including numerous colour- 

 less belonites), being imbedded in a clear material, which (as in parts 

 of those specimens) is a mass of crystallized felspar (without any 

 separate microliths of the same) often showing the characteristic 

 twinning of plagioclase. 



When engaged in putting together these notes I learnt from Miss 

 Raisin that she had visited the north-west quarry at Liinburg. She 

 has kindly allowed me to examine her specimens, and slices from three 

 of them. The resemblance between these rocks from the original 

 locality and those described above is so close, that a very brief descrip- 

 tion will suffice. The minerals occurring porphyritically are the same 

 in all. As regards the groundmass : that of a moderately vesicular 

 specimen from about the middle part of the crag is much darkened 

 with opacite and augite microliths, but the clear interstitial material 

 (not abundant) appears to be a glass. Here and there dark belonites 

 (? a pyroxene encrusted with a brownish iron-oxide) occur, arranged 



1 It affects only the exterior, or penetrates into some of tlie cracks. 



