412 Professor Bonney — On Limburgite from Sashach. 



In eight other analyses quoted by Eosenbusch as representing 

 limburgites we find that 



The percentage of Si Oj ranges from 40-20 to 44*54 



„ AUOj ,, 8-66 ,, 14-89 



„ MgO „ 6-80 „ 13-34 



„ alkalies „ 3-50 „ 8-88 ^ 



But the analysis of a very typical peridotite (dunite) is : * Si Og, 

 39-61 ; AI2 O3, 1-68 ; Fe 0, 8-42 ; Mg 0, 42-29 ; Na^ 0, 0-01 ; K2 0, 

 0-02 ; Ha 0, 5-89. Total, 97-92 (trace of Ca 0). Among the other 

 analyses of peridotites,^ we find that the percentage of Si Og is often 

 a little higher and that of Mg rather lower, the former rising to 

 about 45, the latter falling even as low as about 20, when, however, 

 the Ca O and Fe usually increase, so as to bring the total of the 

 three protoxide bases above 40. The chemical composition of a rock 

 consisting mainly of olivine (i.e. a peridotite) obviously cannot differ 

 materially from that of an olivine ; a slight rise in the silica per- 

 <!entage will indicate the incoming of enstatite, or, with addition of 

 Ca 0, of a monoclinic pyroxene ; when the percentage of AljOg is very 

 low, that is probably present in a spinellid, but with a higher rise 

 (say above 4 per cent.) biotite* or white chlorite* or anorthite may 

 be expected. Thus, as I pointed out several years ago,® limburgite 

 must be much more closely related to the picrites than to the 

 peridotites, and I suggested that it should be regarded as a glassy 

 form of that group.' 



None, however, of these authors name felspar or a felspathoid as 

 a constituent of limburgite, though some hint at the possibility of 

 their being present, so that it may be worth while to put on record 

 a demonstration of the fact which I obtained in the Summer of 1895. 

 I then collected two varieties of limburgite from a large heap by 

 the roadside, approaching the village of Sasbach, rather more than 

 a mile from the southern quarry at Limburg.^ One was much less 

 vesicular (or amygdaloidal) than the other, and looked less likely to 

 have a vitreous groundmass, so that on my return to England I had 

 it sliced. Microscopic examination proved it to contain a considerable 

 quantity of felspar, and I should have published a description of it 

 ^t once had I obtained it from the quarry.^ So I waited in the hope 



^ Generally under 5-5. 



2 Wads worth : " Lithological Studies," p. xxiv. 



^ These remarks do not apply to the list given by Professor Eosenbusch, ut supra, 

 p. 165, but from personal knowledge I must refuse to admit either the Schriesheim 

 picrite or kimberlite into the peridotites. 



* As in the mica-peridotite of Kentucky. 



5 As in the Eauenthal serpentines. Here the alumina only amounts to 1-35 : see 

 C. A. Eaisin, Q.J.G.S., 1897, pp. 251, 257. 



6 Pres. Add. Geol. Soc. : Q.J.G.S., 1885, p. 69. 



' This is virtually admitted by Eosenbusch ("Elemente," p. 361, and " Mikro. 

 Phys.," p. 813). Zirkel (iii, 76) will not allow even this, and uses limburgite as 

 a synonym for magma -basalt. 



* Professor Steinmann writes ' Limberg ' for the place. 



^ It is not my custom to be satisfied with specimens thus collected. But I had 

 been unable to get a carriage at Eiegel (as I had been led to expect), and thus had 

 been_ obliged to go on foot. It was a hot afternoon, a long and fatiguing walk, and 

 my time was limited by trains, so that I had to turn back without reaching the hill. 



