202 GEOLOGY. 



of extreme consolidation by cementation; the iron ore is the product 

 of metasomatism, effected by ground waters; while other phases of 

 metamorphism are due to the heat of intruded rock. It is not to be 

 understood that the metamorphism of any considerable body of rock 

 is effected by any one process alone. Dynamic action, which seems 

 on the whole the most important force in metamorphism, always gener- 

 ates heat, and high temperature, especially in the presence of water, 

 facilitates change. So, too, in the case of igneous intrusions, there 

 is often great dynamic action as well as great heat, and water, the 

 agent of chemical change, is always present. 



Sequence of events elsewhere. — A consonant but not necessarily 

 identical series of events was probably in progress about every other 

 Archean area in North America, and in the world, during the Protero- 

 zoic era; but it does not follow that about every other Archean land 

 area three great systems of rocks were laid down during this long era. 

 About some such areas there may well have been two instead of three 

 systems of Proterozoic rocks, while about others, continuous sedimen- 

 tation may have been in progress from the beginning of the Huronian 

 period to the end of the Keweenawan. Each Archean land area, espe- 

 cially if distant from others, may have had an independent history, 

 though the great deformative movements were probably general. It 

 is probable therefore that adjacent Archean areas were affected by 

 the same sequence of events, though not necessarily to the same extent. 

 The greater the separation of regions, the less closely are their histories 

 likely to correspond. 



Proterozoic Rocks in Other Regions. 



Pre-Cambrian sedimentary formations occur in many other parts 

 of North America x in relations to the Archean similar to those already 

 described. On the whole, it may be said that they resemble the rocks 

 of the Proterozoic systems about Lake Superior as closely as could 

 be expected under the general principles already enunciated. In many 

 cases they appear in surface association with the Archean, but in some 

 cases no such relation exists. 



There are several ways in which the areas of exposed Proterozoic 



1 For references, see foot-notes, pp. 204 to 215 



