108 



rock as the mother-rock from which the serpentine had been produced. He 

 especially insisted on the importance of the accessory minerals as throwing 

 light on the origin of any particular mass of serpentine. Thus one or more 

 of the minerals, chromite, picotite, pleonaste, chrome-diopside, enstatite and 

 pyropp, are found in the typical peridotites. The occurrence of any of these 

 minerals in a serpentine is therefore strong evidence that it has been produced 

 by the alteration of an olivine rock. 



In 1867, TSCHERMAK (1) published two papers in which he confirmed 

 and extended the conclusions of SAXDHERGER. To understand fully the 

 importance of TSCHERMAK'S work, it must be remembered that at the time 

 it w T as executed the opinion that serpentine might be produced by the 

 alteration of almost any rock was generally held. Grabbro, eklogite, diabase, 

 diorite, grauulite and even granite were supposed to be capable of 

 transmutation into serpentine. This view, advocated by BISCHOF (2) and 

 G. ROSE, (3) was based on the fact that gradual transitions from these rocks 

 to serpentine might be observed in the field. Now a transition from one 

 rock to another in space may or may not imply a corresponding transition in 

 lime. It has been shown in the present work that one and the same rock- 

 mass may vary considerably in its miiieralogical composition. An oli vine- 

 rock without felspar may pass gradually into a rock containing a considerable 

 amount of felspar and comparatively little olivine. If the olivine in such a 

 rock-mass becomes serpentinised, then we may have a gradual passage in 

 space from serpentine to a felspathic rock ; but such a transition in no way 

 implies that the serpentine has been produced by the alteration of a felspar- 

 bearing rock. In the first paper TSCHERMAK proved that olivine is much 

 more widely distributed as a rock-forming constituent than the earlier 

 observers had supposed ; in the second he discussed the relations of serpentine 

 to eklogite, granulite and gabbro, and showed clearly that, in the cases which 

 had come under his notice, the serpentine had originated in consequence of 

 the alteration of olivine ; and that the observed transition in space was 

 merely due to an original variation in the composition of the rock-mass. 



The work of SANDHERGER and TSCHERMAK produced a strong reaction 

 against the extreme views of BISCHOF and ROSE, and petrographers naturally 

 asked themselves whether any of the serpentine rock-masses had been 

 produced in any other way than by the alteration of olivine-rocks. That 

 serpentine as a mineral could be produced by the alteration of enstatite, 

 diallage, and non-aluminous augite and hornblende was admitted. But 

 could serpentine as a rock be produced in the same way ? 



The application of the microscope to the study of serpentine, appears to 

 have been first made by AYEBSKY (4 ' ; but it was not until TSCHERMAK took up 

 the question that any important advance was made in our knowledge of the 

 minute structure of the rock. TSCHERMAK showed that the serpentine derived 



(1) liber die Verbreitung des Olivin in den Felsurten. S.A..W., Vol. LVL. p. 261. 



Uber Serpentiiibildung. Same Vol., p. 283. 



(2) Chemical and Physical Geology. Eng. Edit. Vol. IT., p. 415. 



(3) P.A., LXXXIL, p. 511. 



(4) Z.D.G.G. 1858, p. 277. 



