VAEIOLITIC DIABASE OF THE FICHTELGEBIRGE. fil 



began to crystallize out around various points ; the plagioclase 

 needles forming radiating clusters, between wliich theaugite granules 

 were wedged in ; as these half-formed varioles were rolled over, 

 still other layers of the variolitic constituents were de[)osited around 

 them ; when the varioles were originally in close contact, these 

 later layers enclosed several and built up the compound varioles. 



The close connection of the typical variolite with spheroidal struc- 

 ture, which is the case in the Alps, in Italy, in Saxony, and in 

 Wales, reminds one of the recent theory of Prof, de Stefani *, attri- 

 buting the formation of the varioles to secondary decomposition in the 

 outer layer of diabase spheroids. In fact, just as Thomson and 

 other early authorities regarded the spheroidal structure itself as 

 due to weathering. Prof, de Stefani considers these spherulites as 

 due to the same cause. But this theory of the formation of the 

 great spheroids has been generally abandoned since Prof. Bonney's 

 paper on the subject; while the objections to this hypothesis are 

 still more weighty when it is applied to these smaller structures. 

 The fact that the varioles often occur, not on the extreme edge of 

 the spheroids, but often 20 to 30 mm. from it, in rock which is 

 comparatively unaltered, while, where decomposition has gone on 

 along cracks or fissures, it has not produced any such structure, 

 would be conclusive evidence against this view, even were not the 

 analogy between these Palaeozoic varioles and the spherulites of 

 recent lavas sufficiently exact to demonstrate their community of 

 origin. 



YII. SUMMAKY OF CONCLUSIONS. 



1. That the variolitic diabase of Berneck, which may be taken as 

 a type of those of the Pichtelgebirge, is intrusive into the Devonian. 



2. That the variolitic structure occurs in two different arrange- 

 ments : — 



(a) On the surfaces of spheroidal masses of compact diabase, 

 which are comparable to those of the eruptive rock of 

 Mont Genevre. 

 (h) As a true contact-product on the selvage of the diabase. 

 The latter are comparatively rare, and the varioles less perfectly 

 developed. 



3. That the varioles are true spherulites, and not included frag- 

 ments of the Devonian rocks. 



4. That, though the varioles be the product of rapid cooling, too 

 sudden a solidification of the diabase may prevent their formation. 



5. 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. 



6. That the " pseudocrystallites " are rifts and fissures due to 

 contraction ; and that the remarkable optical properties described by 

 M. Michel-Levy are due to the fiUing-up of cracks by felspathic 



* 0. de Stefani, " Le rocce eruttive dell' Eocene siiperiore nell' Apennino," 

 Boll. Soc. geol. Ital. vol. viii. no. 2 (1889), 1890, p. 223. 



