66 Prof. T. G. Bonney — On the RanentJial Serpentine. 



the Eauentlial (Vosges), and has been investigated by Herr Weigand.* 

 According to the latter author, as summarized by Mr. Teall,^ this 

 serpentine " has been formed by the alteration of a hornblende rock, 

 and the change has been accompanied by the removal of lime and 

 the development of a chlorite, in which the original alumina of the 

 hornblende is retained." 



Such an origin for a serpentine vs^ould of course be perfectly 

 possible, supposing that a certain amount of silica was also removed; 

 for the formation of the mineral serpentine from malacolite and other 

 non-aluminous varieties of pyroxene is familiar; but inasmuch as 

 masses either of pure hornblende or pure pyroxene^ are certainly 

 very rare, we should expect such a serpentine to be extremely local 

 in its occurrence, and (like the pyroxene-serpentine) rather abnormal 

 in aspect. Hence it will be well for the student of " British Petro- 

 graphy " to remember that, if Herr Weigand be correct, the origin 

 assigned to the Eauenthal serpentine is not likely to interfei-e with 

 the general rule that normal serpentines result from the alteration 

 of peridotites. But is Herr Weigand quite correct, and does not his 

 statement require a little limitation ? I confess that his paper has 

 awakened some suspicions in my mind which are confirmed by the 

 examination of specimens, procured by Mx'. Teall from Stijrtz of 

 Bonn, and by him kindly placed at my disposal. 



First, as regards the results of Herr Weigand's studies of the rock. 

 In his paper, which is a most conscientious piece of work, he gives 

 us, in addition to an analysis of the white mica-like constituent — a 

 chlorite — an analysis (A) of the soluble portion of the rock {i.e. 

 practically all but the chloritic constituent, not generally abundant), 

 a bulk analysis (B) of the whole rock, and an analysis (C) of the 

 amphibolite into which the serpentine is said to pass. These are as 

 follows : — 



100000 99-625 100-368 



Now a glance at A and B shows that either the one or the 

 other would be a very fairly normal analysis for an ordinary 

 serpentine.* As usual the silica and magnesia are nearly equal in 

 amount ; there is about the ordinary proportion of iron oxides and 

 water. The percentage of lime, though not large, would lead us to 

 suppose that augite or hornblende rather than enstatite had been 



1 Tschermack, Min. Mitt. 1875, p. 175. 2 British Petrog. p. 112. 



^ I find that the diallage rock at Lendalfoot, descrihed by me, Q.J.G.S. vol. xxxiv. 

 p. 778, is not quite free from olivine. This too is a comparatively small mass. 



* That is to say, a rock which has resulted from the alteration of a rock consisting 

 of olivine with a certain amount of a magnesian bisilicate. A serpentine formed 

 from a perfectly typical dunite should have MgO in excess of SiOo. 



