- 



Fibrous serpentine is first produced and the axes of the fibres Ho 

 parallel to the surfaces from which the alteration has proceeded. Owing 

 to parallelism in the arrangement of the individual fibres this serpentine 

 gives a faint but definite reaction with polarised light, and extinguishes 

 when the axes of the fibres lie parallel with the short axis of one 

 of the nicols. The colour of the serpentine is either yellow, yellowish 

 green or green. In most cases the formation of serpentine in this 

 way is accompanied by the separation of iron oxides (ferric oxide, hydratcd 

 ferric oxide or magnetite) along the original cracks. Tf the process has 

 stopped short of the entire alteration of the original mineral we then have 

 granules of oil vine lying in a net-work of serpentine, as in the accompanying 

 figure, and the boundaries of the original olivine grains may be determined by 

 observing the areas over which the isolated granules give simultaneous extinction 

 or depolarise in a uniform manner. 



If the process of serpentinisatioii is complete, so that no trace of the 

 original olivine remains, it usually happens that the finally developed portions 

 of serpentine, occupying the centres of the meshes, are isotropic. 



It must not be supposed that serpentine is the final or only product of 

 the alteration of olivine. Carbonates, silica and limonite are sometimes 

 produced and represent a further stage of alteration. In the altered picrites, 

 described by TSCHERMAK, the olivine crystals are represented by pseudo- 

 morphs composed of calcium carbonate (sometimes 40 p.c.) and a basic 

 silicate containing alumina, lime, magnesia and alkalies. Here there must 

 have been a great transference of material ; magnesia and iron must have 

 been removed, and lime, alumina and other substances introduced. 



Another very interesting feature which sometimes arises in connection 

 with the alteration of olivine is the formation of needles of a colourless 

 hornblende (tremolite or grammatite). Such needles have been described 

 under the name ofpilitc in a pseudomorph after olivine by F. BECKE. (I) They 

 lie confusedly in a chloritic or serpentiuous substance, and from the form 

 of the pseudomorph there can be no doubt that they arise in connection 

 with the alteration of olivine. Similar needles occur in the olivine 

 pseudomorphs of the picrite of Clicker Tor, near Menheniot, Cornwall, and 

 are referred to in the description of the figure of that rock (Fig. II., 

 Plate II.) Precisely similar needles may sometimes be observed in the 

 serpentinous matter of certain hornblende-picrites, as for example, in that 

 of the St. David's boulder. In certain felspar-bearing rocks, either belonging 

 to the picrites or more or less allied to them, (2) the olivines are seen to be 

 surrounded by a zone of hornblende-needles which are arranged in a radial 

 manner with reference to the surfaces of the olivine. These are regarded 



(1) T.M.M. Neue Folge, Vol. V., p. 163. 



(2) As for example the olivine-norites of Olme in Sweden (TORNEBOHM. N.J., 1877, 

 p. 383) ; the olivine-gabbro of Langenlois in Lower Austria (BECKE. T.M.M., 1882, 

 Neue Folge, Vol. IV., pp. 355 and 450) ; the Troctolite of Belhelvie, Aberdeenshire 

 (BONNET. G.M., 1885, p. 444); the oliviue-norite from Lake St. John, Prov., Quebec 

 (ADAMS. Am. Naturalist, 1885, p. 1087) ; and the hornblende-picrite of Peekskill (WILLIAMS. 

 A.J.S., 1886, p. 35). 



