#20 QUARTZ ROCtf. 



as subordinate to the micaceous schist. In many 

 places indeed this latter substance is altogether 

 watitmg; large tracts of country, exhibiting strata 

 of some thousand feet in thickness consisting 

 solely of quartz rock ; giving it - thus a decided 

 claim to be ranked as a principal member of the 

 primary clasis. 



As Micaceous schist and clay slate hate beeb 

 shown to alternate, -thus quartz rock also alter- 

 nates with this latter substance. The m6de of 

 this alternation, like that of the former, is various ; 

 the laminae being of greater or less thickness. 

 On a small scale, these alternations are riot un^ 

 frequent ; between large beds they are rare. In 

 some of these ases the same gradation occurs 

 between tHose Wo substahcesv as in the alterna- 

 tions of micaceous schist with quartz rock ; but 

 in 5 'others* tHey are Separated fcy a c precise line. 

 Of this latter variety; Jura and : Scarba offer strik- 

 ing examples ; flttl qtikrfi rdckr possessing a welK 

 marked and irniepetident character, and forming 

 a principal member of the whole; while the clay 

 slate and micaceous schist alternate with it? 



A more interesting alternation is f 'fiat of quartz 



