58 MR. J. -SV. GEEGORT ON THE 



into the glass, as in many of the Fichtelgebirge variolites, the con- 

 traction due to the segregation of its constituents has not tended to 

 the production of concentric cracks, but of some which are either 

 irregular or parallel to any structural planes, such as lines of flow, 

 that there may have been in the rock : these primary fissures 

 would of course be connected by cross cracks ; thus would arise such 

 a reticulate series as is shown by Prof, von Giimbel *. 



Y. The Relations oe the Diabase. 



In Prof, von Giimbel's descri|)tion clear evidence is given of the 

 intrusive nature of the diabase into the Devonian. The long rib- 

 like dykes below the Schloss, the baked shales and marmorized 

 brecciated limestones of the Miihlleite, the irregular nature of the 

 junction, with the long tongues of the diabase running out into the 

 Devonian, the inclusions of the baked sedimentary rocks, and the 

 absence of tuffs, leave no doubt as to the truth of this view t. In 

 connection with the last f)oint, however, it is necessary to examine 

 the grit-like rock found at the crag by the path to Heinersreuth, 

 and a similar rock with large shale fragments on the south bank 

 of the Oelschnitz opposite Stein, which are no doubt the schal- 

 steins mentioned by von Giimbel. The latter rock is well exposed 

 in a couple of ribs of rock running down the steep slopes of the 

 bank of the river ; it has a coarse gritty matrix, charged with 

 large angular fragments of shale. Microscopic examination entirely 

 dispels the idea that it is a true tuff ; it has, however, been made 

 up almost entirely by the decomposition of igneous and volcanic 

 rocks, and thus chemically, and at first sight microscopically, 

 resembles a tuff. The matrix of the rock is composed of frag- 

 ments of a fine-grained diabase, rolled cleavage-fragments of 

 plagioclase, and fragments of quartz and quartz-mosaic, &c. ; in 

 addition there is a good deal of quartz with a remarkably well- 

 developed micropegmatitic structure. The same constituents, but 

 without the coarse shale fragments, occur in the grit of the crag. 

 In both cases the fragments are mostly somewhat rounded and 

 water-worn, and the rocks are no doubt true grits, the materials of 

 which have been derived from the denudation of an area containing 

 such rocks as those which form the highlands of the Fichtelgebirgc 

 to the south, or of tlie Miinchberg gneiss massif to the north ; in 

 the latter Prof, von Giimbel has described a micropegmatite very 

 similar to that which is so striking a feature in these grits. 



As to the date of the intrusion but little can be said. The 

 diabase can be seen intrusive into the Lower Devonian on the flanks 

 of the Badleite, into the Middle Devonian on the Miihlleite, and into 

 the Upper Devonian in several places along the right bank of the 



* Geogn. Beschr. Fichtelgebirg. fig. 31. 



t Brauns has recently described a variolite forming a crust over a diabase- 

 lava stream at Homertshansen : '" Mineralien unci Gesteine aus dera bessischen 

 Hinterland," Part ii. Section 4; Zeitscbr. deut. geol. Gesellscb. vol. xli. (1890), 

 pp. 502-532. 



