Geological Society of London. 85 



The predominant rock is a coarse-grained foyaite which is found 

 everywhere in loose blocks about the margins of the mass, but not 

 extending beyond it. In the numerous cuttings in the immediate 

 vicinity, dykes of phonolite and basic eruptives (augitite) are ex- 

 ceedingly abundant, foyaite never appearing in a dyke form. There 

 is, however, abundant evidence that foyaite and phonolite are but 

 different phases of the same magma. 



Aside from the dyke phonolites, true effusive plionolites associated 

 with fragmental eruptives (tuflfa) were found high up in the crater- 

 like valley, proving that the mass was a volcanic centre in the most 

 restricted sense of the word. 



This conclusion affords an explanation of some of the peculiarities 

 of the foyaite, which has many characteristics of effusive eruptives 

 mingled with those of the deep-seated ones [Teifengesteine). These 

 have, aside from the porphyritic structure, a scMieren structure 

 revealed by a peculiar fluted weathering (illustrated by a photograph) 

 and the presence of pseudo-crystals in the form of leucite. 



Statigraphically the Tingua foj'aites lie in sheet-like masses like 

 lava-flows, extending from the higher to the lower portions of the 

 mountain, the underlying gneiss being revealed at nearly all levels, 

 wherever the mass has been scored by streams. The general frag- 

 mentaiy character of the rock seems to be due to the undermining of 

 these sheets. 



Specimens and photographs illustrating the peculiar pseudo- 

 crystals in the form of leucite that occur in both the foyaites and 

 phonolites of Tingua (although no leucite has been detected in the 

 rock) were exhibited and discussed. 



2. " The Variolitic Diabase of the Fichtelgebirge." By J. Walter 

 Gregory, F.G.S., of the British Museum (Natural History). 



The author has examined the variolitic diabases in the neighbour- 

 hood of Berneck, and adduces evidence of their having been intruded 

 into the Devonian rocks before the latter were affected by the great 

 earth-movements which have folded the PalEeozoic rocks of the 

 district. He finds that the variolitic structure occurs in two different 

 arrangements : (a) on the surfaces of spheroidal masses of compact 

 diabase, which are comparable with those of Mt. Genevre ; (b) as a 

 true contact-product on the selvage of the diabase, the latter being 

 comparatively rai'e, and the varioles less perfectly developed. 



He gives proofs that the varioles are true spherulites, and not 

 fragments of Devonian rocks, as supposed by von Giimbel. He 

 argues that though they are the product of rapid cooling, too sudden 

 a solidification of the diabase may prevent their formation, and that 

 for a similar reason the amygdaloidal is less variolitic than the 

 compact diabase, the loss of the water that occupied the vesicles 

 having diminished the fluidity of the rock. Finally, he maintains 

 that the "pseudo-crystallites" are rifts and fissures due to contrac- 

 tion, and that the remarkable optical properties described hj Michel- 

 Levy are due to the filling- up of cracks by felspathic matter deposited 

 in optical continuity with the crystalline fibres on each side. 



