QUARTZ ROCK. 321 



rock with graywacke schist, and in this instance 

 its analogy with the secondary sandstones which 

 so often follow that substance, is particularly ap- 

 parent. Jura, Scarba, and Lunga, afford con- 

 spicuous examples of this association. 



It is also found associated by alternation with 

 primary sandstone, which it so much resembles 

 in many important particulars ; but, in this case, 

 the alternations appear always to be on a large 

 scale, or, at least, none have occurred within my 

 observation where it can be considered as subor- 

 dinate in quantity. 



Peculiarities of structure in quartz rock are 

 very rare, but, where they occur, they are in- 

 terejsting from their resemblance to those which 

 are found in the secondary sandstone. Thus it 

 often contains the indications of spheroidal, and 

 of a cylindrical, or minutely columnar, imbed- 

 ded structure, exactly resembling that> hereafter 

 described at more length in the account of those 

 rocks, but far less distinct, and less frequently 

 exposed by the action of the weather. In a few 

 very rare instances also, of which an example is 

 found in Glen Tilt, it contains prolonged and' 



