Professor Bonnet/ — On Limburgite from Sasbach. 411 



PI. symmetrica to 95 mm., PI. atlenuata to 98 ram., and PI. rostrata 

 from 80 to 100 mm. 



Amongst recent Pleurotomce there are more than a dozen species 

 which exceed Pi. prisca in length of shell, up to PI. grandis from the 

 India and China seas, which attains to a length of 135 mm. 



Although Mr. Hugh Beadnell's Pleurotoma prisca does not " break 

 the record " for size amongst Eocene species, he may at least claim 

 to have unearthed one of the largest specimens of this genus ever 

 found at Barton. 



IV. — On the Limburgite from near Sasbach. 

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



SO much confusion has existed on the subject of limburgite, that, 

 although it has now been partly dissipated, a record of my own 

 investigations may be of use, at any rate to English petrologists. 

 The rock has been classed by some with the peridotites. From 

 these it is separated by Rosenbusch,^ who in his latest work places it 

 in a subgroup with the augitites in proximity to the basalts. Zirkel 

 uses limburgite as a synonym for magma-basalt. F. Graeff ^ implies 

 relationship with the nepheline-basalts. Barker's ^ remarks suggest 

 similar conclusions, as do those of Cole,* though his opening words 

 are perhaps slightly misleading. All, however, lay so much stress 

 upon the absence of felspar, by giving such definitions as "Limburgite 

 und Augitite sind die felspathfreieu Entglieder der Gesteinreihe 

 Trachydolerit, Tephrit," etc.,* or " Frei von Feldspar und Felspath- 

 oiden ist der Limburgit," "^ and in some cases by reference to the 

 peridotites, that students may readily overlook the fact that this 

 rock is only free from felspar in the same sense as tachylite or many 

 pitchstones and obsidians. 



A little consideration of the original analysis should have shown 

 that the rock could not possibly be placed in the same group as the 

 peridotites. This (to which I add a more recent one) gives :— 



SiO, 



TiO; 



AI2O3 



Fe^Os 



FeO 



MuO 



MgO 



CaO 



NaoO 



Ka'O 



H,0 



1 "Elemente der Gesteiaelehre," p. 361. 

 massigen Gesteine," p. 811. 



2 " Mitt, der Grossher. Badischen Gaol. Landesanstalt," ii, p. 405. 



3 "Petrology for Students," 1895, p. 179. 



4 " Aids to Geology," 1898, p. 262. 



5 Rosenbusch : " Elemente," p. 361. . 



8 Graeff : loc. cit. * From a pamphlet by Professor Steinmann. 



7 " Eleniente," p. 363. ^ Traces also of Cu and Ni. 



