DETERMINATION OF ROCKS 65 



majority of cases they are of aqueous origin, that is, they 

 have been laid down as sediment in water. Their com- 

 ponent pebbles and grains are therefore usually more or 

 less rounded and water- worn (Fig. 9). But minute 

 crystalline particle . from older rocks are apt to 



ir angularity, as fine sand does which, swept 

 along in suspension by a river, undergoes no attrition on 

 the way. The coarse varieties, consisting of compacted 

 , arc termed wiigfomtraits when formed of rounded, 

 when formed of angular fragments. These 



Fio. MX -Piece of volcanic tuff. 



coarse-grained rocks pass into grits and sandstones, where 

 the materials, usually more or less siliceous, have been 

 reduced to the condition of sand Sandstones are in the 

 majority of cases composed of grains of quartz, which 

 are often well rounded, and when examined with a 

 strong lens or a microscope look like large rounded 

 boulders. Argillaceous rocks are those composed of the 

 muddy or clayey sediment, sometimes arranged in laminae 

 of deposit, as in shaU, at other times with no f: 

 as in fireclay and mxdstone. An important series of 

 fragmentaJ rocks has been formed by the consolidation 



9 



