from near Bert rich, in the Eifel. 245 



The results afforded by fusion certainly go to prove that Roth 

 was correct in his statement that the Bertrich glass is melted augite 

 and not olivine, although in the hand-specimens appearances are 

 strongly in favour of its being the latter. That the glass owes its 

 origin to any secondary heating, is very improbable ; for it is difficult 

 to conceive of any causes in a lava- flow that would bring about 

 a local rise of temperature, without affecting the ground-mass of the 

 rock and the minerals enclosed in it, especially as the rock itself 

 fuses so quickly and thoroughly. Neither do I think that the 

 formation of the glass can be due to the rapid cooling, under little 

 or no pressure, of melted augites, which were earned along on the 

 surface of the lava stream. In that case it would be impossible to 

 explain the occurrence of totally unaltered augites lying side by side 

 with this glass. I found the latter in hard and compact lava, not at 

 all slaggy in appearance, with distinct signs of columnar structure 

 set up in it ; hence the cooling down must have taken place slowly. 

 Prof. Bonney has suggested that the pieces of glass are fragments 

 of some deep-seated magma almost agreeing in composition with 

 pyroxene, which, indeed, occurs as a rock. 1 These were carried up to 

 the surface in the liquid lava from some subterranean reservoir, and 

 cooled at first very rapidly and in a different manner to the augites 

 in the current. Partial devitrification may possibly have taken 

 place owing to changes of temperature at some stage before the final 

 solidification of the mass, but I think it is more probable that the 

 enclosed microliths may have formed, owing to local circumstances, 

 when the glass actually became solid. The pieces of augite in the 

 glass with ragged edges and altered appearance were doubtless 

 entangled in it while it was still in a liquid condition. The little 

 rings in the glass formed by the microliths are probably due to the 

 latter collecting round some of the numerous vesicles, for, as I have 

 already stated, the glass is very vesicular. I think it is thus 

 possible to explain the occurrence of unaltered augites and olivines 

 lying immediately next to the glass in the ground-mass of the rock, 

 and the non-intermingling of the latter with the glass. 2 Von 

 Dechen's objection, viz. the non-occurrence of the glass in other 

 Eifel lavas, does not seem to me to carry much weight, as one would 

 not necessarily expect to find fragments of a deep-seated magma 

 (and this one of rather exceptional composition) in every flow. He 

 apparently found melted and unmelted augites lying side by side in 

 the lava. This has not been my experience, but as he concludes his 

 description of the rock by saying that microscopic examination 

 would probably lead to the further clearing up of difficulties, 3 it is 

 reasonable to suppose that he did not carry out such an examination, 

 which would doubtless have considerably influenced his opinion. 



I cannot conclude this brief communication without acknowledging 

 my indebtedness to Prof. Bonney for his kindness in examining my 

 hand-specimens and slides, and for his valuable advice. 



1 Viz. pyroxenite and hornblendite. 



2 Miss C. A. Kaisin, B.Sc, has kindly shown me a piece of tachylitic glass 

 occurring in a dark leucitic lava of unknown locality. 



3 Loc. cit., p. 26. 



