I.] ROCK-FORMING MINERALS 17 



~ Quartz, the crystalline form of silica, is found 

 massive and in veins in the primitive rocks, and in 

 fragments of all sizes in the granites, gneisses, and 

 similar rocks. From the waste of these crystalline 

 rocks are derived the sandstones of all geological ages 

 and directly or indirectly the sands now existing. 

 In a sandstone rock the grains of quartz are bound 

 together by a cement, which may be oxide or car- 

 bonate of iron, as in the Lower Greensand of Surrey 

 and Beds, and in some of the Wealden sandstones, or 

 carbonate of lime, as in the Kentish Rag, or even silica 

 itself, as in the hard blocks of tertiary sandstone, which 

 are left as " grey wethers " on the surface of the chalk. 

 In some of the older sandstones the rock is practically 

 homogeneous ; heat, pressure, and solution having 

 thoroughly felted the grains together. Many sand- 

 stones weather rapidly, through the solution of the 

 cement binding the grains together ; the resulting sand 

 has the same texture as it possessed before it was 

 cemented into a rock. 



The grains of sand that are first weathered from a 

 crystalline rock possess an angular shape, but are 

 soon rubbed down in running water into rounded 

 grains with a surface like fine ground glass. Hence 

 the degree of angularity which the sand grains show gives 

 some indication of the amount of wear and tear they 

 have suffered since their origin as sand. Below a certain 

 size, however, quartz grains seem no longer capable of 

 rubbing against one another, but remain angular even 

 after long travel in running water. Daubree has shown 

 that angular fragments of sand of less than 01 mm. 

 in diameter will travel in water without becoming 

 rounded, hence any rounding 01 smaller grains of sand 

 must have been due to solution. 



Silica in the crystalline state is very slightly soluble 



B 



