Geology of the Harz Mountains. 233 



Gabhro and Plagiodase-enstaiite rocks are found in great 

 masses between the granite of the Brocken and that of the 

 Ockerthal. In the Eadauthal above Harzburg, the gabbro 

 is largely quarried for road metal. It is there made up of 

 labradorite, hypersthene, and diallage, with titaniferous iron, 

 magnetic pyrites, quartz, biotite, etc., as accessory con- 

 stituents. Veins of graphic granite traverse the gabbro at 

 places, and gabbro veins traverse the granite of the Eckerthal 

 above Harzburg, showing both rocks to be contemporaneous. 



Plagioclase-enstatite rocks occur in the gabbro in thin 

 isolated zones, running from S.W. to KE. They consist, 

 when fresh, of anorthite, enstatite, and occasionally olivine, 

 and can be arranged in a series, at one end of which the rock 

 is made up almost entirely of anorthite, and at the other of 

 enstatite. The olivine varieties have sometimes taken up 

 water and become serpentinised, while the enstatite crystals 

 remain entire, and give the rock a peculiar, semi-metaUic, 

 lustrous appearance. This variety of Schillerfels has been 

 named bastite, from the Baste, a place in the forest, near the 

 top of the Eadauthal, where it is found in considerable quantity. 



c. Postgranitic Eruptive Rocks. 



Quartz-porphyry occurs in dykes and interbedded sheets in 

 the Eothliegendes at Lauterberg. The dykes have a grey 

 to reddish brown ground-mass enveloping large orthoclase, 

 quartz, mica, and pinite crystals, while the sheets have few 

 crystals, and weather in platey fragments like shales. 



The quartz-porphyry of the Auerberg, near Stolberg, is a 

 great " stock " or boss, with cream-coloured, grey, or greenish 

 ground-mass, and abundance of quartz dihexahedra — the so- 

 called Stolberg diamonds — which are very characteristic of 

 the rock. Externally this rock resembles the quartz-porphyry 

 from the coulee of South Corriegills, Arran, which is also full 

 of double pjrramids of quartz. The Arran rock is, however, 

 much darker in colour than any of the Auerberg quartz- 

 porphyry I have seen, and it shows fine flow-lines on 

 weathered faces. The quartz-porphyry of the Auerberg 

 shows signs of incomplete fusion with the sedimentary rocks 



