416 Professor Bonney — On Limhurgite from Sasback. 



in a sort of ' fern-leaf ' pattern. The groundmass of a second 

 specimen (almost free from vesicles) taken not far from the bottom 

 of the cliff, very closely resembles those I obtained in 1895. The 

 smaller augite microliths are not numerous, and all the other 

 constituents are embedded in a clear crystalline groundmass of 

 plagioclase felspar. In a third specimen, generally similar in 

 structure, taken from the base of the cliff, aggregated granules of 

 augite occur locally in a similar groundmass, in which I think 

 a little nepheline is also present. Thus, felspar is abundant in 

 much of the rock at both ends of the hill, of which the original 

 limburgite, with the base of brown glass, is only a local condition.^ 

 According to a section of the Limburg Hill from north-west to 

 south-east, published by Professor Steinmann in a pamphlet (for 



Fig. 2. 



1. Upper flow, limburgite (spheroidal). 



2. Upper tuff. 



3. Middle flow, nepheline-basalt. 



4. Lower tuff (with wood, etc.). 



5. Lower flow, hlack limburgite. 



6. Upper flow, limburgite (without olivine). 



7. Upper tuff. 



8. Middle flow, limburgite (with phillipsite, etc.). 



9. Lower tuff. 



10. Lower flow, ? limburgite or nepheline-basalt. 



11, Loess. 



From the top of the hill to the bottom of the southern quarry is about 20G feet. 



a copy of which I am indebted to Mr. Haas), three flows, parted by 

 tuffs, are exposed in each quarry. He speaks of the top one in the 

 northern quarry as limburgite, the middle as nepheline-basalt, the 

 lower as black limburgite.^ In the southern quarry, from which 

 the specimens described in this paper are taken, the upper is 

 limburgite without olivine, the middle limburgite, the lower 

 limburgite or nepheline-basalt. Of the specimens here described, 

 Nos. 1, 2, and 3 are from the ' middle limburgite,' Nos. 4 and 5 

 from the ' lower stream.' I may say that during my examination of 

 these rocks (before I read Professor Steinmann's notes) I was on the 

 look-out for nepheline. Most of the groundmass is indubitably 

 felspar, but one or two small crystals included in that, and a little 

 interstitial mineral (neither very well preserved), are very suggestive 

 of that mineral, which the analysis would lead us to expect. 



These notes, I hope, will make it clear to English readers that the 

 typical limburgite (like tachylite) is only a local glassy condition 



1 Miss Raisin informs me that the specific gravity of a compact specimen is 3-058. 



2 Herr F. Graeff (loc. cit.) also gives a section, naming the top and bottom flows in 

 each limburgite and the middle one nepheline-basalt. He notices the different colour 

 of the mass at the bottom of the southern quarry, and says this has a different habit 

 from typical limburgite. 



