Geology of the Harz Mountains. 231 



sheets and bosses in the Devonian and Hercynian series, 

 both on the Upper and Lower Harz. There are two varieties, 

 one of which is compact and the other granular. The 

 granular diabase, or diabase gabbro, as it is sometimes called, 

 is a crystalline mixture of labradorite and augite or diallage 

 with magnetite, titaniferous iron, and apatite, and is coloured 

 green by a chloritic decomposition product. The fine-grained 

 or compact variety has the same constituents, but is often 

 characterised by the development of chlorite and calcite in 

 spherules, when it is known as " Blatterstein." The plagioclase 

 is occasionally porphjrritically developed in crystals half an 

 inch in length, when the rock passes into a diabase- or 

 labradorite- porphy rite. 



The diabase of the string ocejpTialus horizon, both on the 

 Upper and Lower Harz, is associated with much pale green 

 or bluish fine-grained tufP or schalstein. A very good section 

 of the diabase and tuff beds of the Upper Harz is exposed in 

 the bed of a small stream which runs into the Hutthal about 

 3J kilos S.E. from Clausthal. The volcanic beds have also 

 been cut through in making the road between Clausthal and 

 Lerbach, but the section is now becoming rather obscured. 



Orthoclase porphyry ^ or Syenite porphyry occurs, according 

 to Lossen, as interbedded sheets along with the diabase of 

 the Elbingerode basin. It has a fine-grained or compact 

 ground-mass, with large crystals of felspar in Carlsbad twins, 

 which has been ascertained to be of the variety named 

 microperthite. 



Kersantite ^ or Micadiorite was discovered by Lossen at 

 Michaelstein a short distance westward from Blankenburg. 

 Its relations to the surrounding rocks are obscure, but Lossen 

 has placed it in the group of pregranitic eruptive rocks. 



It is' characterised by abundance of plates of biaxial mica 

 not exceeding 1 or 2 mm. in diameter, and contains much 

 orthoclase and plagioclase. Lossen considers the ground-mass 

 to be orthoclase. Phlogopite, apatite, rutite, garnet, cyanite, 

 sillimanite, and zirkon are accessory ingredients. The 



1 Lossen, Zeitschr. der deutsch. geol. Ges., Bd. xxxii., 1881, p. 175; 

 xxxiv., 1882, p. 199. 



2 Lossen, Jahrb. d. k. preuss. Landesanstalt, 1880, p. 22. 



