Measuring Processes of the Past 21 



to these questions, which are assumed to hold for past times 

 as well as for the present, we try to rebuild the past. This is 

 illustrated by what we know about the weathering of rocks. 



Weathering 



Many of the stone buildings in our cities and towns show 

 the effect of changes in weather and temperature. Certain 

 of the softer red sandstones scale off very quickly. A hard 

 crystalline rock, such as granite or marble, endures much 

 longer than sandstone or a porous rock. When water soaks 

 into the tiny cracks or pores of a rock and then freezes, as 

 on cold nights, the expansion of the ice makes pieces of the 

 rock split off. 



Waves beating upon the rocks on the shore will pound 

 the hardest granite to pieces. In its descent as rain, water 

 absorbs carbon dioxid from the air and comes to act as a 

 weak acid, which is a solvent for many minerals. The wind 

 carrying moisture or particles of sand also tends to grind 

 down rock. The effect of this process is sometimes seen in 

 the appearance of the window glass of a lighthouse which 

 is etched by the wind-driven particles of sand in a severe 

 offshore storm. If we assume that similar processes have 

 been going on in the past, we not only account for facts that 

 are readily observed, but we have also a means of measuring 

 how rapidly important changes have taken place in the past. 

 Recognizing the impossibility of getting direct observations, 

 and our need of depending upon inference from observed 

 facts, we may still speak with some confidence as to what 

 has happened. 



Measuring Processes of the Past 



Any one of us, looking at the Niagara Falls today, can 

 see the reasonableness of the idea that the edge of the rocks, 

 over which the water plunges, was farther down the valley 

 ten years ago or fifty years ago than it is today. Of course 



