232 Proceedings of the Royal Physical Society. 



kersantite of Michaelstein contains 54*25 per cent, of SiOg 

 and 16-09 Al^Og. 



h. Granitic and Related Eruptive Rocks. 



More recently von Groddeck has described another 

 occurrence of kersantite at Lautenthal on the Upper Harz.^ 

 The rock occurs here as a well-marked dyke, which runs 

 southwards through the Culm greywackes from the edge of 

 the Palaeozoic series at a point a short distance westward from 

 Langelsheim. It has a length of about 5|^ miles (8 kilo.), and 

 is continually shifted in a lateral direction by a series of 

 faults so as to resemble on the map a flight of irregular steps. 

 This kersantite has the peculiarity of having a percentage of 

 silica, which varies between 46 and 63*6, and is nearly 

 inversely proportional to quantity of carbonates contained 

 in the rock. The fact of the dyke traversing the Culm strata, 

 and being cut by the faults which intersect them, leads me 

 to class it along with the members of the granitic group 

 which bear the same relation to the surrounding rocks. 



Granite occurs at three places as intrusive bosses — (1.) 

 Brocken, (2.) Eamberg, (3.) between the Ockerthal and 

 Eadauthal, and at each locality is similar in composition. 

 It consists of reddish orthoclase, greenish oligoclase, quartz, 

 and black mica. White mica does not occur in the Brocken 

 granite, and only sparingly at the other places. Tourmaline 

 is found as an accessory ingredient in considerable quantities 

 along with other minerals. The granite sends out veins or 

 apophyses into the adjacent rock at many places. One of 

 these runs from the granite of the Eamberg westwards 

 through the Bodethal, and has been found by Lessen to 

 possess a granitoid centre with porphyry sides, which show a 

 true glassy base. The edges of the granite at Andreasberg 

 and elsewhere show a passage into porphyry due to the more 

 rapid cooling at the line of contact with the sedimentary 

 rocks. 



Amphihole granite, quartz diorite, augite diorite, etc., occur 

 on the east side of the Brocken, at the edge of the granite 

 near Schierke — the most inaccessible corner, of the Harz. 

 1 Jahrb. d. k. preus. Landesanstalt, 1882, p. 68. 



