246 Professor Bonne// — Parent-rock of the Diamond. 



II. — The Parent-rock of the Diamond. Reply to a Criticism, 



By Professor T. G. Bonney, D.Sc, LL.D., F.R.S. 



A SHORT time since Mr. G. Trubeubacb, Managing Director of 

 tlie Newlands Diamond Mine, West Griqualand, drew my 

 attention to a notice by Professor Beck ^ of tbe paper wbicb I read 

 to tbe Royal Society last June, on tbe Parent-rock of tbe Diamond 

 in Soutb Africa, kindly sending me a translation of tbe original. 

 As Professor Beck's criticisms are founded on sucb a travesty of my 

 publisbed opinions^ tbat I can only suppose bis knowledge of our 

 language to be very imperfect, and as tbe bypotbesis wbicb he 

 advances appears to me untenable, I ask permission to reply to 

 tbe one and point out tbe improbability of tbe otber. 



1. Professor Beck objects to my terming tbe crystalline garnet- 

 pyroxene rock, wbicb sometimes contains diamonds, an eclogite, 

 remarking,^ " wbicb, bowever, must raise doubts, as tbis name 

 has hitherto been applied only to a similar mineral combination of 

 crystalline schists (schiefer), while Bonney particularly emphasizes' 

 the eruptive nature in this instance." In my paper I took tbe 

 precaution of stating tbat I was well aware doubts bad been 

 expressed in tbe past as to tbe origin of eclogite, but that after 

 good opportunities of studying it I was convinced it was an 

 intrusive igneous rock. Tbat sucb doubts should have existed is 

 not surprising. Tbe significance of foliation was for long imperfectly 

 understood, and as eclogite is generally associated with crystalline 

 schists, and is not seldom foliated, it was supposed to have had 

 a similar origin. In such cases it is a metamorphic rock, but 

 not in tbe sense formerly implied by tbat word ; tbe foliation (by 

 no means universal) being only a superinduced structure, wbile 

 tbe evidence of intrusion is sometimes conclusive. Eclogite formerly 

 was left with sundry otber little understood rocks in a kind of 

 'oddment' drawer.* Does tbis uncertainty in the past as to the 

 genesis of a rock debar us from using its name afterwards when 

 we are able to do it with more precision ? Or does Professor Beck 

 mean to say that the whole history of a rock, as well as its mineral 

 composition and structure, must be known before we can give it 

 a name? In both tbe latter this South African rock is truly an 

 eclogite, and I maintain tbe name may legitimately be used for it, 

 when ill-grounded uncertainties as to tbe genesis bave been cleared 

 up. What does Professor Beck mean to do with the olivine rocks 

 (or peridotites) and tbe corresponding serpentines? There also 

 a similar doubt as to origin formerly existed. 



2. Tbis, bowever, is a mere difference of opinion on a question 

 of nomenclature. I pass on to one more important. Professor Beck 

 criticizes, at too great a length for full quotation, my supposed 



1 Zeitschrift fiir Praktisclie Geologie, December, 1899. 



* Proc. Royal Soc, Ixv, p. 223. 



^ I quote from the translation sent to me by Mr. Trubenbacb, though I have 

 occasionally made a slight alteration iu the terms used. 



* See, for instance, Cotta on Rocks, translated by Lawrence, 1878, p. 310. 



