146 THE DAWN OF LIFE. 



is immediately overlaid by the mica-slate formation. 

 Lithologically, this newer division of the gneiss is 

 characterized by the predominance of a grey variety, 

 rich in quartz, with black magnesian-mica and ortho- 

 clase, besides which a small quantity of ohgoclase is 

 never wantinor. A further characteristic of this Her- 

 cynian gneiss is the frequent intercalation of beds of 

 rocks rich in hornblende, such as hornblende-schist, 

 amphibolite, diorite, syenite, and syenitic granite, and 

 also of serpentine and granulite. Beds of granular 

 limestone, or of calcareous schists are also never alto- 

 gether wanting ; while iron pyrites and graphite, in 

 lenticular masses, or in local beds conformable to the 

 great mass of the gneiss strata, are very generally 

 present. 



" In the large quarry of Steinhag, from which I first 

 obtained the Eozoon^ the enclosing rock is a grey 

 hornblendic gneiss, which sometimes passes into a 

 hornblende-slate. The limestone is in many places 

 overlaid by a bed of hornblende-schist, sometimes five 

 feet in thickness, which separates it from the normal 

 gneiss. In many localities, a bed of serpentine, three 

 or four feet thick, is interposed between the limestone 

 and the hornblende- schist ; and in some cases a zone, 

 consisting chiefly of scapolite, crystalline and almost 

 compact, with an admixture however of hornblende and 

 chlorite. Below the serpentine band, the crystalline 

 limestone appears divided into distinct beds, and en- 

 closes various accidental minerals, among which are 

 reddish-white mica^ chlorite, hornblende, trem elite. 



