52 &OCK-METAMORPHISM. 



amorphous mineral, considered by Delesse to be feldspathic and 

 related to halleflinta : this part passes insensibly into a coat of 

 pyrosclerite ; and the latter is surrounded by phlogopite. These 

 lumps have consequently a concentric structure. 



The gneiss, dark in colour, consists of layers of opaque ortho- 

 clase, translucent plagioclase, dark bronzy mica, and dark green 

 hornblende : quartz is more or less irregularly intermixed with 

 these minerals ; also, as accessories, garnet, pyrites, graphite, 

 and sphene. Where the gneiss and hemithrene are in immediate 

 contact, their respective minerals are intermixed ; nevertheless 

 both rocks are well differentiated, and readily distinguished 

 from one another. The hornblende unmistakably changes into 

 pyrosclerite, as is also the case with the augite ; and the least 

 reflection must make it clear that the two minerals plagio- 

 clase and malacolite have contributed to produce the cal- 

 cite : besides, according to Delesse, the dark bronzy mica of 

 the gneiss has become magnesiated into the lighter-coloured 

 phlogopite which characterizes the hemithrene. Moreover it is 

 highly probable that the calcite, where it is the dominant con- 

 stituent, has been increased by carbonate of lime brought in 

 by water from extraneous sources. 



Figure 1, Plate VI., represents a section, about 13 feet in 

 height, showing the gneiss and hemithrene in contact, as seen 

 on the right-hand side of the quarry near its entrance. The 

 beds of gneiss (coloured red in the figure) , it will be seen, come 

 in between two masses of bedded hemithrene (coloured green) 

 rudely jointed. There is no appearance of a fault bringing the 

 two rocks into their present relative position; nor, considering the 

 perfect continuity between their respective beddings (which, it is 

 to be remarked, have a uniform dip), can it be said that the hemi- 

 threne is in detached masses that have been let into a synclinal 

 hollow and by this means preserved in their present position. 



The bedding of the hemithrene is in other parts of the quarry 

 somewhat obscure ; and so is the limestone, especially where the 

 interposed concretions are present. 



Delesse, who had favourable opportunities for examining the 

 geology of the Vosges, where the hemithrene occurs, states that 

 the " calcaire saccharoide est toujours completement enveloppe 

 par le gneiss dans lequel il forme des flambeaux irreguliers ou 

 lenticulaires tels que ceux qui ont ete signales dans la Scandinavie 



