THE STUDY OF THE ROCKS 



up into sand, and the sand again becomes so finely 

 divided that it forms mud. From this it is clear 

 that the composition of the sand depends largely 

 on the nature of the rocks in the neighbourhood. 



First of all, we must place a little of our sand 

 upon a piece of black paper and examine by re- 

 flected light. We shall soon notice that sand is 

 anything but the simple gritty looking substance it 

 appears on the sea shore. At least four different 

 substances are certain to be seen; if the sand be 

 clean, we shall see bright glassy crystals of quartz; 

 angular, sharp-edged pieces of flint; minute, flat, 

 glistening plates of mica and the broken remains of 

 shells. Should our sand be taken from a spot in 

 the vicinity of volcanic rocks, it will probably con- 

 tain opaque pieces of magnetite; we may not 

 recognise these when we see them, but they may 

 be separated from the rest of the sand by means 

 of a magnet, to whichT they will adhere. Probably 

 our sample of sand will be dirty or stained with 

 iron, in this event we may wash it with weak 

 acid, then after drying, we may examine by 

 transmitted light and the various crystals will 

 show up well. 



Let us take some clay as our next example. 

 Clay is the substance from which the rocks known 

 as shale and slate have been formed. We cannot 

 see much of the structure of clay with our micro- 

 scope so let us wash it and, by so doing separate 

 it into its components. We must put a little clay 

 into a tumbler of water and stir it vigorously for 



129 I 



