204 Miss C. A. Raisin — A Hornhlende-Picrite from Valais. "^ 



as if moulded along the cleavage planes, and sometimes within 

 serpentine. Pyrites occurs within the enstatite and the hornblende. 



As in other picrites, certain minerals act as a ground mass, which 

 ■varies even in one slide. The order of consolidation seems to have 

 been — (1) olivine, (2) pleonaste, (3) augite and hornblende or 

 enstatite, (4) a second variety of enstatite, now steatitic. 



The rim bordering the olivine grains is occasionally penetrated 

 by serpentine or chrysotile. This is deposited apparently along 

 cleavage planes in narrow spaces which extend beyond as forked 

 branching canals, suggesting a more irregular fracturing. The 

 canal part reminds us somewhat of an ' eozoon ' structure, as was 

 remarked by Professor Bonney in noticing a similar development in 

 a troctolite.^ It is also like the structure in gelatine figured by 

 Professor Sollas,^ and similar to specimens which I have obtained, 

 in some simple experiments. In the last case, at least, the cause 

 seems to be a cracking of the material, the cracks often having 

 certain more regular cavities as starting-points. Similar canals are 

 found in the slide here described along rather regular cleavage 

 planes in part of a crystal which resembles bastite, and some 

 of the dusty decomposed patches of that mineral resolve them- 

 selves in a thin slice under a high power into a system of what 

 look like very fine tubules. In a gabbro from Portsoy, Professor 

 Judd describes radiating cracks as caused by the •'' expansion of 

 the olivine during its hydration," and injected with serpentine.^ 

 These cracks are generally well seen in the troctolites, and 

 Mr. Harker refers to this character in the Lleyn picrite.^ In the 

 Zmutthal specimen, a definite and limited rim around the olivine 

 seems to be penetrated, thus producing a more regular foi'm of 

 a canal system.* As to the origin of the structure, while the 

 expansion from hydration may have acted as one cause, it seems 

 probable that the mode in which the mineral originally crystallized 

 (possibly from its narrow area) developed certain planes of 

 weakness, not at first recognizable, along which corrosion or fracture 

 could afterwards act. 



The boulder which I am describing is probably connected with 

 other rocks from this neighbourhood, although it is distant from 

 them. One of these contains unchanged olivine — the olivine-gabbro 

 of the Matterhorn^ — in which the occurrence of green pleonaste 



^ " On Bastite Serpentine and TroktoKte in Aberdeenshire " : Geol. Mag. 1885, 

 Decade III, Vol. II, p. 444, note. 



- Trans. Eoyal Irish Acad., vol. xxix, pp. 462-3. 



3 Q.J.G.S. 1886, vol. xlii, pp. 94-5, pi. vii, fig. 7. 



* Q.J.G.S. 1888, vol. xliv, p. 455. 



^ In my specimen of the Peuarfynydd picrite, which I collected in 1885, the 

 olivine contains, sometimes at the edge, clusters of needle-shaped structures either 

 parallel or radiating in a stellate form, the needles in both cases suggesting minute 

 fissures starting from a centre. At places the parallel bundles seem to be connected 

 with a fibrous chrysotile development 



® T. G. Bonney, "On some specimens of Gabbro from the Pennine Alps": 

 Min. Mag., April, 1878. T. G. Bonney, " Petrol ogical Notes on the Euphotide 

 or Saussurite-Smaragdite Gabbro of the'Saasthal" : Phil. Mag. 1892, vol. xxxiii, 

 ser. 5, p. 237. P. W. Schafer, Tschermak Min. and Petr. Mitth. Wien, Bd. xv, 

 Heft. 1 and 2, 1895, p, 132. 



