CHAPTER V. 



ROCKS OF G-ROUP A. 



THE first rock of this group which appears to have attracted general 

 attention is the olivine-rock described by LELIEVRE, in 1787. It was 

 subsequently named Lherzolite by DE LA METHERIE (1) from the locality 

 (Lake of Lherz) in the Pyrenees whence it was obtained. In 1862, DAMOUR (S) 

 showed that two-thirds of this rock consists of olivine, and that 

 DE LA METHERIE, who regarded it as mainly composed of augite, had, 

 therefore, been mistaken as to its composition. The remaining constituents 

 are now known to be enstatite, chrome-diopside and chromite, picotite or 

 pleonaste. According to the researches of CHARPENTIER and MARROT the 

 rock occurs in a liassic limestone which has been rendered crystalline by the 

 intrusion of granite. Some discussion has taken place as to whether the 

 rock is truly igneous or not, but the balance of evidence is decidedly in 

 favour of the former view. DBS CLOISEAUX (3) has observed similar rocks 

 in the Silurian limestone of Eaux Bonnes (Basses Pyrenees) and in the 

 granitic region of Beyssac (Haute Loire). 



One of the most remarkable eruptive olivine-rocks is that of the Dun 

 Mountain in New Zealand described by Hoc:HSTETTER. (4) The rock in question 

 is composed of olivine and chrome-iron ore. It forms the top of a mountain 

 4,000 feet high, and stands in the closest relation with a huge sheet of 

 serpentine, eight miles long and one or two broad, which is intrusive in 

 mesozoic strata according to HOCHSTETTER. This rock has been named Dunite by 

 its discoverer. 



Olivine-rocks frequently occur associated with the crystalline schists. Such 

 rocks have been described by SANDBERGER, (5) TSCHERMAK, (6) KJERULF, (7) 

 PETTERSEN, (8) DATHE, (9) GUMBEL, (10) AXEL ERDMANN, (11) BROGGER, (12) 



(1) Theorie de la Terre, Vol. II., p. 281. 



(2) B.S.G., II. Ser., Vol. XIX., p. 413. 



See BONNEY. The Lherzolite of the Ariege. G.M. Decade II., Vol. IV., 1877, p. 59. 



(3) B.S.M., Vol. XIX., p. 48. 



(4) Z.D.G.G., 1864, p. 341. See also Reise seiner Majestat Fregatte Novara, Geologischer 

 Theil, Band L, 218. 



(5) N.J., 1866, p. 391. 



(6) S.A.W., Vol. LVL, (1867). 



(7) Verhandlg. d. Wiss., Gesell. Christiania, 1864. 



(8) N.J., 1876, p. 613. 



(9) N.J., 1876, p. 225. 



(10) Geog. Beschr. d. Fichtelgebirges. Gotha, 1879, p. 148. 



(11) N.J., 1849, p. 837. 



(12) N.J., 1880, Band II., p. 187. 



