60 MR. J. W. GREGORY ON THE 



daloids. Prof, von Giimbel has described * the diabase sheets of 

 the Labyriuthenberg as coated with variolite ; but, though I 

 hammered carefully over the whole section, I could find no true 

 variolite, but plenty of an amygdaloid that somewhat closely re- 

 sembled it t. 



Though the variolite is thus shown to be a true endomorphic 

 alteration-product, and due, no doubt, to contraction during a some- 

 what rapid cooling, the view that it is an ordinary contact-selvage 

 is not fully adequate, as frequently around some of the Devonian 

 masses in the Miihlleite no variolite occurs at the junction. It is 

 only where the diabase is at the same time spheroidal that the 

 variolitic structure has been fully produced. In the Fichtelgebirge, 

 as at Mont Genevre and in Saxony, the variolite mainly occurs, not 

 as a contact-alteration-product along the junctions of the diabase 

 and other rocks, but on the surfaces of great diabasic spheroids. 

 At Mont Genevre, in one or two places, a thin spherulite film does 

 occur along the margins of the diabase dykes, but this is rare and 

 always minute. Similarly, in this locality, where the variolite 

 occurs as a contact-product it is thin and inconspicuous, and the 

 varioles are less perfectly developed. In such cases the solidification 

 has apparently been too sudden to allow of the segregation of the 

 felspathic and pyroxenic constituents into spherulites. The variolitic 

 structure has been due to rapid cooling, but is of a less extreme type 

 than that which has ]iroduced the amorphous glass of normal basic 

 selvages. 



These considerations further suggest the explanation of the rarity 

 of the variolitic amygdaloid, as compared with the compact varieties 

 of the rock. When, owing to diminished pressure or other cause, 

 the solidifying diabase became vesicular by the explosion of its 

 water into steam, the amount of water in the molten magma would 

 be lessened ; this would consequently become less fiuid, and the 

 solidification of the rock be further hastened. 



That the variolite appears most perfectly in association with 

 spheroidal masses is quite natural if we regard the latter structure 

 as due to processes of contraction during solidification, as shown by 

 Prof. Bonney in his well-known paper J. 



The diabase in this knoll, rapidly cooling by the conduction of its 

 heat to the neighbouring rocks, contracted into great spheroids, 

 M'hich, while semiviscid on the periphery, were still fluid within ; 

 under the pressure of the forces that drove them upwards, these 

 rolled over one another and were drawn out into oval masses. It 

 was during this process that the felspathic and pyroxenic constituents 



* Geogn. Beschr. Ficbtelg. p. 483. 



t Kor did there seem to be any variolite from this locality in the Muteiim 

 of the Bavarian Oberberganit in Munich. This collection contains specimens 

 irom several of the other localities mentioned by Prof. \on Giimbel , and these 

 are certainly true variolites, as, e. g., from Steiubach. For the opportunity of 

 examining the collection there I must express my thanks to Dr. L. von Ammon. 



\ T. G. Bonney, " On Columnar, Fissile, and Spheroidal Structure,"' Quart. 

 Journ. Geol. Soc. vol. xxxii. (187G), pp. 140-154. 



