Geology of the Harz Mountains. 245 



the horizon has been named the " zone of the granular 

 diabases." This fact might lead to the supposition that they 

 are interbedded sheets, but the evidence seems to point to 

 their having never reached the surface, but having been 

 injected into the sedimentary strata either as stock-like 

 bosses or else as sheets nearly coincident with the bedding 

 planes. In this latter respect they may resemble the great 

 Whin Sill of ^Northumberland, which was once believed to 

 be a true interbedded rock because it remains for considerable 

 distances in the same geological position.^ A still better 

 example might be cited in the great diabase sheet which 

 extends from the East Neuk of Fife westwards through the 

 Lomonds and Cleish Hills, then reappears in the Abbey 

 Craig at Stirling, and continues on to Kilsyth, always 

 remaining on the horizon of the Hurlet or '' Carboniferous " 

 Limestone. The distance between the east end of Fife and 

 Kilsyth is over 50 miles — equal to the whole length of the 

 Harz — and the Geological Survey have ascertained that the 

 diabase throughout its whole extent is a distinctly intrusive 

 sheet. 



The facts which point to the intrusive character of these 

 diabases are as follows : 



1. Want of Contimdty. — Had all the little spots of diabase 

 been parts of great interbedded sheets they would have had 

 a more or less continuous outcrop like the contemporaneous 

 diabases of Elbingerode or the Upper Harz. 



2. Absence of Tuff. — There is no tuff associated with any 

 of these diabases as there is with the others, as well as with 

 the Permian volcanic rocks at Ilfeld and Mansfeld. The 

 absence of tuff' — always a sure indication of subaerial origin 

 — is, however, only of significant and not of vital import. 



3. Contact Metamorphism. — The intense alteration in the 

 sedimentary rocks surrounding most of these diabases is 

 direct proof of their intrusive character. Lava streams pro- 

 duce in general very little alteration even on the beds over 

 which they flow. Lyell tells of a lava stream on Etna 



^ See paper by Messrs Topley and Lebour " On the Intrusive Character of the 

 Whin Sill of Northumberland " (Quart. Journ. Geol. Soc, vol. xxxiii., 1877, 

 p. 406). 



