111) 



SiO 2 ... ... ... ... 41-57 



Al.Os ... ... ... ... 0-67 



Fe a O, 2-63 



FeO ... ... ... ... 5-31 



CaO ... ... ... ... 1-22 



MgO ... ... ... ... 86-66 



CO, ... ... ... ... 0-51 



Loss 11-88 



100-45 



Microscopic examination showed that the rock is mainly composed of a 

 mineral giving either lath-shaped sections or else sections in the form of 

 irregular plates. The former gave straight extinction and showed a fine 

 striation indicating a perfect cleavage of the mica type, parallel with the 

 broad flat surfaces. They were sometimes seen to be distributed irregularly 

 and at others to be arranged in two parallel series intersecting approximately 

 at an angle of 90. These observations show that the mineral in question 

 occurs in irregular plates and possesses a strongly marked basal cleavage. 

 It has the hardness of talc. The other constituents of the rock are 

 magnetite, in the form of grains, and the diallage already referred to. 



Rocks of a similar character are described from other localities in the 

 Alps. DRASCHE'S researches leave some doubt as to the true character of the 

 talc-like mineral and also as to the true nature of the rock by the alteration 

 of which the serpentine has been produced. It will presently be shown that 

 the later researches of HUSSAK supplement those of DRASCHE, and furnish a 

 solution of the problem relating to the origin of these rocks. 



In 1877, Professor BOXXEY (1) gave a detailed account of the field 

 relations and microscopic structure of the serpentines of the Lizard, and 

 proved that they are altered olivine-rocks allied to the Lherzolite of 

 Ariege (2j of which he published a description in the same year. He has 

 since investigated other British serpentines, but, as a detailed account of his 

 researches will be given when the rocks are described it is unnecessary 

 to do more than to refer to them in this connection. 



We have now to notice a paper by WEIGAXD on the serpentines of the 

 Vosges, which appeared in 1875. (3 ' Many of these serpentines, as for 

 instance those of BOXHOMME and STARKEXBACH, are shown by their micro- 

 scopic structures and accessory minerals to have been formed from olivine- 

 rocks. One, however, viz., that of the Rauenthal, is exceptional and deserves 

 a more careful description on our part. The rock in question occurs in a 

 gneissic series, certain bands of which are rich in mica and hornblende. By 

 the increase in the latter mineral the rock passes into an umphibolite. The 

 serpentine is associated with the latter rock. As the serpentine is approached 

 the hornblende of the amphibolite becomes lighter in colour and shows a 

 platy structure. The cleavage faces assume a sub-metallic lustre. Small and 



(1) Q.J.G.S., vol. XXXIIL, p. 884. 



(2) G-.M. 1877. p. 59. 



(3) T.M.M.. ; 1875, p. 183. 



