68 NATURE STUDY .\BY GRADES 



Even a drop of the latter poured upon a piece of limestone 

 will produce effervescence. This indicates limestone. An 

 odor similar to burnt powder, where two rocks are struck 

 together, 'indicates a silicate, generally the hardest of com- 

 mon rocks. 



If possible, visit a cliff and a stream and gather fragments 

 of the talus from the former and pebbles from the latter. 

 Compare their shapes. Why are the rocks recently broken 

 from the cliff angular and irregular, while the pebbles are 

 rounded? Discuss erosion, especially as seen in our moun- 

 tain streams. 



Experiment. Fill a strong bottle with clean pebbles, 

 washed until the water used is perfectly clear. Then shake 

 them vigorously with the clear water for one minute or more 

 before the class, and note how dirty the water becomes. Dis- 

 cuss how the water became muddy. Pass it through a filter, 

 and show how much dirt was ground off in only one minute. 

 Consider the effect of continuous wear. 



Describe a marble mill where toy marbles are made from 

 stones. The stone from which marbles are to be made, is 

 first broken into little blocks, all of about the same size. 

 These blocks are placed in a large cylinder, or drum, which 

 is made to revolve day and night while a stream of water 

 passes through it. The pieces of stone within the cylinder 

 are thrown against each other, much as pebbles in a stream, 

 and in time they wear away all corners and edges, and when 

 taken out are almost perfect spheres. In the marble mill 

 only one kind of stone is used at a time, and the pieces, 

 being uniform in size and hardness, wear away in a more 

 uniform manner than the pebbles in a stream, where all 

 kinds and sizes are mixed, and the wearing motion is very 

 irregular. 



