Pro/! T. G. Bonneif — On the Rauenthal Serpentine. 67 



associated with the dominant olivine of the original rock, that is, 

 that it had been one of the varieties of peridotite, which approach 

 nearest to picrite, and by some would even be included under that 

 name.' If we can take the amphibolite as in any way representing 

 the parent rock of B, we see that not only must we remove a con- 

 siderable amount of lime (almost enough, so far as figures go, to 

 make up the difference in water), but we must get rid of a consider- 

 able proportion of alumina, and a large amount of silica.'^ 



If these substances have been removed, it is a little strange that 

 we do not find traces of them in the form of veins and similar 

 deposits in the rock itself. But Herr Weigand does not mention 

 any, and I find none in the specimens before me. It may, however, 

 be objected to this mode of reasoning, that the amphibolite does not 

 fairly represent the rock from which the serpentine (B) was formed, 

 but only a rock which was closely related to it ; and, having 

 regard to the " badly mixed " condition of many of the basic igneous 

 rocks — such as the peindotites and picrites — the objection is not un- 

 T'easonable. Let us then see whether the difficulty would disappear 

 if we were dealing with some other variety of hoi-nblende rock. 

 Clearly we must select as its constituent mineral a hornblende poor 

 in alumina, rich in magnesia. In all the analyses quoted by Dana 

 in his Mineralogy, the percentage of silica (as we should expect in a 

 bisilicate) is greatly in excess of magnesia. Even antholite, which, by 

 its richness in iron and poorness of lime, would be the most suitable 

 constituent of the mother-rock of the Eauenthal sei'pentine, has at 

 most SiOa : MgO = 584: : 314.^ Thus a consideration of Herr 

 Weigand's analyses alone suggests that either the specimens analyzed 

 or the specimens examined microscopically were abnormal, or there 

 had been some slight error in the one or the other investigation. 



I proceed now to the result of my own examination of Mr. Teall's 

 two specimens. Slides have been cut from each, two from one (that 

 which he kindly gave me), one from the other. After careful study 

 of these, while I agree with Herr Weigand that the original rock has 

 contained a variety of hornblende, and that some of this has been 

 altered into a serpentinous mineral, I can come to no other con- 

 •clusion than that the dominant constituent in each is a serpentine 

 formed from olivine. 



On examining one of the slides cut from my specimen,* we see 

 that it is chiefly composed of two constituents rather irregularly dis- 



1 For myself I think it better to apply the term picrite to the rather indefinite 

 group which is intermediate between the true peridotites (or felsparless rocks where 

 olivine dominates) and the olivine-dolerites. Olivine + augite and olivine + horn- 

 blende need names. 



" Suppose, tor purposes of comparison, we assume that no magnesia has been 

 jeraoved. In (B) AI2O3 : MgO = 14 : 360 ; in (C) AI2O3 : MgO = 67 : 263. 

 These proportions expressed in decimals are -038 and -254. Again in (A) SiO, : MgO 

 = 369 : 360— almost equal; in (C) Si02 : MgO = 464 : 262— about 17 : lo". 



2 Anthophyllite would also be a very suitable mineral, but it is evident that we are 

 not dealing here with a rhombic mineral, but one which much resembles tremolite. 



* These happen to be slightly thicker than Mr. Teall' s slide, so that the distinction 

 •of the constituents is rather more conspicuous. 



