THE HARDENING OF ROCKS 



still warm, and when the climate of the earth might be 

 described as a continual thunderstorm. In this period 

 earthquakes still had a great deal to do with the forma- 

 tion of the rocks, but then, as now, the sea and lakes and 

 oceans laid them down. Geologists call this the Proterozoic 

 Era. There are great masses of these Proterozoic rocks in 

 North America. In Arizona the three periods of rock 

 formation are sometimes visible together, and may, 

 indeed, be perceived in some of our photographs ; the 

 Archaean all jumbled together being the lowest ; Protero- 

 zoic lying crumpled or tilted over them, and the later 

 rocks resting more regularly on these strata. In America, 

 however, these separate ages of the Proterozoic rocks can 

 be identified, and each age is represented by rocks thou- 

 sands of feet in thickness. Three separate ages of rocks 

 are found in this great era in North America. It is not 

 very important to remember their names, which are merely 

 those of the localities where these great deposits are most 

 marked, but it is important not to forget that each of 

 these depositions of rocks represents a period in the 

 earth's history older than the lifetime of a river or a lake 

 and as old as the lifetime of a continent. The lowest of 

 these divisions consists of rocks that are much altered by 

 the heat of the rocks below. The topmost division is 

 hardly altered at all. In Scotland we have similar rocks. 

 The Torridonian sandstones, 8000 to 10,000 feet thick, are 

 believed to belong to this era. In France also, in Spain. 

 Germany, Finland, Sweden, India, and Brazil, the Protero- 

 zoic rocks are found. In the lowermost of them are no 

 signs that living things ever existed, but in the upper ones 



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