Geology of the Harz Mountairis. 253 



kojDf and the border of the range, outliers of Zechstein are 

 seen resting on the old rocks themselves without any red 

 rocks between, which proves that the Permian strata had 

 here been elevated and subjected to considerable erosion be- 

 fore the deposition of the Zechstein. The case in which the 

 quartz-porphyry dykes run along lines of fissure in the Her- 

 cynian series has been already referred to. These dykes, as 

 well as the fissures, are abruptly truncated by the Zechstein 

 at several places, so that the interbedded volcanic rocks which 

 were probably connected with the dykes must have been 

 completely removed before the Zechstein began to form. 



Hercynian pebbles occur in the Zechstein, proving that the 

 Harz was not completely submerged at the close of the 

 Eothliegendes period. During that long period of subaerial 

 denudation the great series of red rocks at Mansfeld which 

 sometimes exceeds 3000 ft. in thickness was deposited. It 

 is, however, unlikely that the whole of the region now known 

 as the Harz remained bare up to the Zechstein period, for it 

 is probable that the eastern end of the area between Mans- 

 feld and Ballenstedt was entirely covered by the red rocks, 

 outliers of which have been left at Opperode near Ballen- 

 stedt and at Biesenrode near Mansfeld. 



During the Eothliegendes period the Harz was surrounded 

 by a ferruginous sea in which there was little life, but 

 when the Zechstein age began an abundant fish fauna was 

 introduced, and the fine mud which was being deposited all 

 round the southern part of the district became mixed with 

 particles of bituminous matter from the decomposing organ- 

 isms in the water. Small quantities of metallic matter 

 carried in suspension or solution were reduced and acted on 

 by the hydrosulphuric acid and other gases evolved during 

 the decomposition, and were deposited along with the bitu- 

 minous matter and clay, chiefly as sulphides of the respective 

 metals. The richest part of the shale contains about 3 per 

 cent, of copper and -^^ per cent, of silver. 



Now, the question suggests itself, where did these metals 

 come from ? Salt water and most felspars have been found 

 to contain small quantities of the precious metals, and when 

 felspathic rocks are eroded away their metallic ingredients 



