takk] common rocks 23 



ing. This is a wide view into the past to be gained from an 

 uninteresting looking piece of rock, is it not? Suppose we find a 

 metamorphic rock, say a schist, in which there are beautiful crys- 

 tals of garnet. What does it suggest? It tells us that the rock 

 was buried miles below the surface of the earth where the pres- 

 sure was so great that the original rock from which the schist was 

 formed was crushed and broken and new minerals formed. And 

 then the earth's crust was folded and broken and mountains were 

 formed and after a long period of erosion the rocks were exposed 

 as we find them today. 



The ro.cks as we find them exposed at the surface are full of 

 joints and cracks. Water gets into these openings and dissolves 

 out some of the material and carries it away. In winter the 

 water freezes and this helps to widen the joints and cracks and 

 causes the rock to crumble and break up. The summer heat 

 expands the rocks then they contract and break and split up. 

 The roots of ]:)lants and trees also get into these openings and aid 

 in prying the rocks apart. Through the mechanical action of 

 the freezing, thawing, etc., and the chemical action of the water 

 and the acids it contains the rock is decomposed. i\Iuch ma- 

 terial goes in to solution and is carried away by the streams to the 

 ocean. Alany minerals are changed, but some, such as quartz, 

 are not attacked by the solutions very nuich. These various 

 products of decay fonu the rock debris of which the upper por- 

 tion is called soil. In some areas that are nearly flat the soil is 

 thick, in other areas that are rough much of it is carried away by 

 the streams. Evidence of this is shown by the muddy streams 

 after everv rain. The forests and grasses aid in keeping the 

 soil on the slopes by holding the particles together with their 

 roots. As soon as the forests are removed the streams begin to 

 remove the soil. In some areas, the Southern Appalachians, for 

 example, thousands of acres are worthless now for this reason. 

 Manv places can be found where the gradation of the unaltered 

 rock into the soil may be seen. The study of this change is very 

 interesting. 



There is a constant but usually very slow movement of this 

 decayed material to the sea. In the sea it supplies the material 

 for the future beds of limestone, shale and sandstone. There 

 are various estimates of the rate at which the surface of the 

 United States is being lowered. A recent estimate by the United 

 States Geological Survey is that one foot is beinsf removed in 

 9,120 years. 270,000.000 tons of dissolved matter and 513,000.- 



