VARIOLITIC DIABASE OF THE FICHTELGEBIRGE. 49 



with chlorite, and a large number of irregular spaces of a slightly 

 dichroic, transparent material. Under a higher power, though the 

 rock is greatly decayed, the principal points in structure can be 

 determined. The groundmass is resolved into three constituents : 

 there is a greenish, isotropic matrix, the transparency of which is 

 diminished by a dust of minute opaque grains ; it also contains a 

 crowd of small granules and crystals ; in most cases these are 

 certainly augite, and it is probable the whole series are of this 

 mineral. Occasionally a few fine felspar needles occur iu the 

 groundmass, and sometimes these are grouped into radial clusters. 

 The small irregular rifts and spaces come out very prominently, 

 owing to the transparency of the mineral that fills them ; this is 

 sometimes slightly dichroic and polysynthetieally twinned, and 

 shows a strong tendency to crystallize in rhomboidal forms. 

 These characters show that it is dolomite, and it is obviously second- 

 arj'- in origin ; most of the smaller lines, however, are only singly 

 refracting, and in this material occur numerous small inclusions. 

 These structures must be compared with the " pseudocrystallites " 

 described and figured by M. Michel-Levy, and further reference will 

 be made to them in a later section of the paper. 



The varioles in this rock belong to the variety with radial struc- 

 ture and sharply marked off from the matrix : they thus differ 

 from those figured by von Giimbel * from the opposite side of the 

 Oelschnitz. The outer layer of the varioles is greatly altered and 

 decomposed, and, while the minerals themselves have been destroyed, 

 the external margin has been corroded. Internally they consist of 

 a large number of small, radiating, and often branching fibres of a 

 grey mineral, which is certainly a plagioclase, and apparently 

 oligoclase ; intercalated between these are granules of augite, 

 similar to those that are scattered through the groundmass. The 

 varioles are of the compound radial type. Fig. 3 (p. 50) shows one 

 with the plagioclase needles radiating from several centres, while 

 around the margin are several smaller secondary varioles. 



The extensive alteration that the rock has undergone, indicated 

 both under the microscope and in the rock by the calcareous veins 

 that traverse the diabase, does not encourage one to expect much 

 assistance from chemical analysis. Prof, von Glimbel gave analyses 

 of the groundmass and varioles f ; that of the latter is the more 

 interesting, but it really does not help much : — 



Silica 64-33 



Alumina 13'46 



Oxide of iron 8-29 



Lime 4*63 



Magnesia 1'58 



Potash 1-75 



Soda 5-36 



99-40 



* Geogn. Beschr. Fichtelgebirg. fig. 31. t Ibid. p. 217. 



Q. J. G. S. No. 185. E 



