50 



MR. J. W. GREGOEY ON THE 



The paucity of alkaline earths and alumina renders it difficult to 

 see what combination of oligoclase and augite could give such an 

 analysis, especially when we have to deduct a little lime and 

 magnesia for the secondary products in the interspaces. The history 

 of this analysis, however, does not dispose one to place much 

 conjS.dence in it, and, with such intensely altered rocks, the micro- 

 scope is probably a safer guide than the chemical balance. 



Pig. 3. — Section of tlie variolitic Diabase, with a large variole ; from 

 the pathway to the Colonnade, Berneck. Some Pseudoerystallites 



Q I 



occur in the lower part of the figure, (x-yr. — N.B. The 



numerator represents the original magnification, and the deno- 

 minator the reduction from the size of the field of the micro- 

 scope.) 



The microscopic examination, however, fully proves the identity 

 of this rock with that of Mont Genevre, though, being older, it has 

 undergone greater alteration ; the pea-shaped bodies or " KnoUchen " 

 are shown to be true varioles, composed of a mixture of felspar 

 fibres and augite granules and needles similar to that which occurs 

 in the typical variety from the Cottian xilps. 



Here, then, in this boss of diabase we have the variolite of the 

 Fichtelgebirge ; but, instead of being a mere contact-product deve- 

 loped at the junction of the diabase and the rocks into which it 

 is intrusive, it occurs in the centre of the igneous rock, at some 

 places at least fifty yards from any other rock. According to vou 

 Gumbel's map this knoll is in the middle of the diabase area ; but 

 on the opposite slope of the Celschnitzthal there may be seen, 

 sandwiched between coarse ribs of diabase, some smooth slopes that 

 indicate the presence of the Devonian *. Pebbles of the same 

 formation may be picked up in the gully on the south side of the 

 knoll up which the path rises from the foot-bridge ; and a few yards 



* These are shown in the woodcut by Prof, vou Giimbel, ojp. cit. p. 525. 



