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from olivine possesses a characteristic structure (masehen-structur) Avhieh, in 

 most cases, enables one to speak confidently as to its origin, even when every 

 portion of the original substance of the olivine has disappeared. 

 The serpeiitinisatiou of the olivine proceeds along the irregular cracks 

 which traverse the mineral. Water obtains access to the interior by 

 means of these cracks and, in olivines containing a considerable amount 

 of iron, the oxides of this metal magnetite and ferric oxide are 

 formed along them. AVheii the serpentinising process is complete these 

 oxides remain, and thus define by their distribution the original structure of 

 the olivine. Even when iron oxides are not formed in this way the micro- 

 structure of the serpentine substance itself stands related to the original 

 structure of the olivine, and leaves no doubt as to the origin of the rock. 

 The irregularity of the network here referred to is of course due to the 

 fact that olivine does not possess strongly marked cleavages. Now the other 

 minerals from which serpentine may be produced possess well marked 

 cleavages, and it might naturally be expected, even anterior to observation, 

 that evidence of this would remain in the resulting serpentine. 



In 1871 Drasche (1 ' communicated a paper " Tiber Serpentin und 

 serpeiitiiialmliche Gesteine." In this paper he showed that serpentines 

 which bear the closest possible reseniblence to each other so far as composition 

 is concerned differ considerably in micro-structure. On the one hand, there 

 are the serpentines with " maschenstructur " the true serpentines of the 

 author and on the other, rocks yielding a true serpentine analysis, but 

 having a totally different micro- structure the " serpentiuahnliche gesteine." 

 Many instances of true serpentines are described by the author, but as 

 they do not differ in any essential respect from the similar serpentines 

 described by TSCHKKMAK, nothing need be said about them in this historical 

 sketch. As a type of the " serpentiiiahnliche gesteine " the author takes a 

 rock from AVindisch Matrey, in the Northern Tyrol, which is conformably 

 interstratified with a calcareous mica-schist. This rock varies considerablv 

 in character and is often traversed by veins of calcite, asbestus and chrysotile. 

 One specimen, of a light olive green colour, flecked with yellowish brown specks, 

 was examined both chemically and microscopically. It possessed a bluish 

 lustre and an uneven fracture. Here and there green scales of a mineral 

 with one well-marked cleavage could be detected, and as flakes of this mineral 

 when examined in convergent polarised light showed an optic axis, it was 

 identified as diallage. The following analysis proves that the rock is a 

 serpentine so far as composition is concerned ; but that it is not a normal 

 serpentine is proved by the fact that it is not acted upon by hydrochloric 

 acid. 



(1) T.M.M. 1871, p. 1. 



