MARCH 179 



impervious to water, while with sand alone the water 

 would pass too quickly through. A mixture of sand and 

 clay is the best for the farmer. 



Out of the clay is made porcelain, china, crockery, 

 flower-pots, bricks, drain-pipes. 



Quartz axd Pebbles: 



Milky quartz in the form of white pebbles may be 

 found in abundance in most streams, and may be bought 

 at aquarium supply stores. Frequently mixed with them 

 are feldspar pebbles which, however, are rather softer and 

 may be scratched, usually with difficulty, Avith a knife. 



All of the quartz pebbles Avill be found to be hard, 

 easily scratching glass, light in weight, with a glassy 

 lustre, transparent or translucent, or even opaque, break- 

 ing irregularly. 



The green color of some of the pebbles is due to plants 

 grown in their former habitat — a stream of water ; and 

 the yellow color of others, to the presence of iron which 

 has rusted. 



All pebbles began their existence as angular pieces 

 of rock. They have become rounded by dint of being 

 rubbed against each other in water. Glacial pebbles, 

 which are common enough in gravel pits and north of 

 the line of the terminal moraine in the ice age, have 

 of course been rounded by being rubbed along by the 

 glaciers of that period. They are often scratched. 



Because of the great hardness of quartz and its lack 

 of susceptibility to chemical change, we seldom have it 

 reduced to anything finer than sand, while feldspar is 

 finally reduced to impalpable clay. 



Quartz crystals are built up on the plan of six, typi- 

 cally a six-sided right prism, capped on both ends with 



