THE STUDY OF THE ROCKS 



gome little time, then still stirring, if we pour c^ 

 the muddy liquid into another tumbler, we shall 

 find that our old friend sand has settled to the 

 bottom. Clay then is merely mud and sand, but 

 we must not throw this sand away without examin- 

 ing it for, very frequently, beautiful minute fossils 

 are to be found amongst it. Little creatures dwel- 

 ling in the mud, become buried in it and, as more 

 and more mud is formed above them, and it becomes 

 partly solidified in the form of clay, they become 

 fossilized. 



If now we examine a piece of sandstone with our 

 pocket lens we shall find that it very closely re- 

 sembles the sand we have already studied. It may 

 be so soft that we can break it up in our fingers, 

 then if we examine the powdered sandstone beneath 

 the microscope we shall need to be experts to tell 

 whether we are examining sand or sandstone. Should 

 our rock be too hard to break up in our fingers the 

 addition of a little weak acid will reduce it to sand. 

 We see now that a large proportion of sand and 

 a small proportion of mud result in the formation 

 of sandstone, whereas if the proportions are reversed 

 and we have more mud than sand the resulting rock 

 is known as shale or slate. In shale we very fre- 

 quently find fossils, in sandstone rarely, for the 

 reason that structures buried in the mud, destined 

 to become shale, are protected from the atmosphere 

 and the action of water; sandstone, on the other 

 hand, is porous, moisture trickles through it and any 

 delicate structures which may have been buried in 



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