ARGILLACEOUS ROCKS 207 



Gravel is composed of rounded, water-worn pebbles, varying 

 in size from a pin-head up to cobblestones and boulders. The 

 coarser kinds are often called shingle. Gravel may be composed 

 of almost any kind of rock material, but the commonest pebbles 

 are of quartz, because of its greater resistance to wear. Masses 

 of quartz will be only rounded into pebbles, when other substances 

 are ground into fine silt, or chemically disintegrated, and so washed 

 into deeper water. 



Conglomerate is a firm rock, made up of pebbles, embedded in 

 a matrix of finer material, very generally sand. As above re- 

 marked with regard to gravel, the component pebbles of a con- 

 glomerate may be derived from any kind of hard rock, but siliceous 

 pebbles are of most frequent occurrence. Different names are 

 given to conglomerate, according to the character of the pebbles, 

 as quartz conglomerate, flint conglomerate t limestone conglomerate, 

 granite co?iglomerate, etc. 



b. Argillaceous Rocks 



These rocks contain a greater or less proportion of clay, but 

 nearly always with large admixtures of other substances, such as 

 exceedingly fine sand, felspathic mud, and the like. The particles 

 of these rocks are extremely fine and are carried for long distances 

 before settling to the bottom. Hence the muds and clays are dis- 

 tributed over wider areas than the gravels and sands, and deposits 

 of them indicate quieter and, usually, but not always, deeper waters 

 than the conglomerates and sandstones. 



Kaolin, also called China or porcelain clay, is a nearly pure 

 white clay, which is formed principally from the decay of the alka- 

 line felspars of granitic rocks. 



Potter's Clay is a somewhat less pure variety, having a consider- 

 able quantity (18 to 37%) of finely divided quartz, and small 

 quantities of lime and iron. 



Brick Clay is a still more impure mixture of sand and clay, with 

 lime, magnesia, iron, potash, and soda. Clays with considerable 

 percentages of iron burn red in the kiln, from the oxidation of 

 their iron compounds into Fe 2 3 . Ordinary red bricks do not 



