286 ELEMENTARY STUDIES OF SOME COMMON ROCKS. 



(4) varieties and uses of clays. 



There are many different kinds of clay : 



China-clay (used in porcelain manufacture), 



Brick-clay (used for bricks and tiles), 



Fire-clay (used for fire-bricks and anything that has 



to resist high temperatures), 

 Terra-cotta-clays, pipe-clay, Fuller's earth, and so on. 



It is the commonest things in nature that are the most 

 useful. 



SAND. 



Material. One or two samples of sand for each pupil, 

 laid out in a watch-glass or on a slip of paper. 



Sands are easily gathered and easily stored; they are 

 clean and easily handled and in other ways very suitable 

 for lessons with children. The teacher will find, if he once 

 begins to make a collection of them, that they vary re- 

 markably in appearance and character. Some are rough 

 and gritty (Peterhead, etc.), some are rather finer (Aber- 

 deen) and show nearly the same constituents as granite, 

 some are crowded with broken or complete shells and 

 other organic contents (St. Andrews), some contain bits 

 of limestone, ironstone, etc. (Scarborough), some have 

 abundant fragments of flint (Yarmouth), some are re- 

 markably varied in colour (Alum Bay, I.W.), some are 

 rich in heavy metallic ores like tin (St. Ives), some have a 

 proportion of very round grains like little balls (Soudan). 



The leading constituents of most sands are quartz 

 (glassy, often clear) and felspar (opaque, more pearly, 

 red and other colours), but not a few contain white mica, 

 and some sands even in this country are nearly entirely 

 made of calcareous matter, and therefore dissolve away in 

 hydrochloric acid. 



If the teacher is not disposed to collect samples during 

 the holidays, he can easily procure them from friends who 

 live in coast towns, or through dealers. 



