Geology of the Harz Mountains. 243 



The oldest rocks are greywackes of moderately coarse grain, 

 then come the clayslates, calcareous and siliceous beds of 

 the Wiederschiefer group, which are covered by the sandstones 

 and quartzites at the base of the Devonian series. Next 

 comes a series of shales, greywackes, and occasional lime- 

 stone beds, ending in the great Upper Devonian limestone 

 mass of the Iberg and Winterberg at Grund. The overlying 

 Culm measures begin with shales and pass into greywacke, 

 the highest beds of which are very coarse and conglomeratic. 



There must thus have been a gradual shallowing of the 

 sea and an approach to land in this area towards the close of 

 the Lower Carboniferous age. The remains of land plants in 

 the Culm tell the same tale, as they are found in greatest 

 abundance in the higher beds but scarcely ever occur in the 

 underlying shales. 



The ancient land from which the components of the Culm 

 are derived was probably a metamorphic clayslate area with 

 many beds of splintery quartzite, and pierced by dykes and 

 bosses of granite and quartz-porphyry, whose fragments now 

 make up the conglomerate at the top of the series. In Silesia 

 and sometimes in Thuringia the pebbles in the Culm grey- 

 wackes can be traced to old rocks in the neighbourhood, but 

 the parent rock of the Harz pebbles is neither to be found on 

 older parts of the Harz itself nor in any of the surrounding 

 ranges, nor are the pebbles ever again seen in the newer 

 conglomerates which flank the hills. There must therefore 

 have been a complete submergence of this part of the old 

 continent during the Coal-measure period. It then became 

 covered up, and has not again been exposed by denudation. 

 The very same kinds of pebbles as are found in the conglome- 

 rate of Grund occur also in the Culm conglomerate of Wal- 

 deck, halfway between the Harz Mountains and Bonn on the 

 Ehine, so that it is not improbable the shore of the sunken 

 district may have extended westwards in that direction.^ 



2. Palaeozoic Volcanic Action. 

 Lossen's map shows abundant patches of diabase distributed 



^ Vide V. Groddeck, Jahrb. d. k. pr. geol. Landesanst., 1882, pp. 65, QQ ; 

 also E. Kayser, Keues Jahrb., 1884, footnote, p. 95, 



