358 PRE-CAMBRIAN PERIODS 



vigorous chemical work upon the heated crust. In the course of 

 ages the surface of the globe became so far cooled and the crust 

 so thick that the earth's interior heat ceased to control the tem- 

 perature of its surface. 



THE PRE-CAMBRIAN PERIODS — I. ARCHiEAN 



It is unfortunate that an account of historical geology should 

 necessarily begin with the most difficult and obscure part of 

 the whole subject, but the treatment must be in accordance 

 with the chronological order, and the oldest rocks are the least 

 intelligible. The ordinary criteria of the historical method, 

 namely, the stratigraphical succession and the comparison of 

 fossils, fail us here almost entirely, and the only way of corre- 

 lating the rocks of different regions and continents is by means 

 of the characters of the rocks themselves. In the present state 

 of knowledge, " lithological similarity " is not a safe guide. So 

 many metamorphic rocks, once referred to the Archaean, have 

 proved to be of much later date, that some cautious geologists, 

 who have no confidence in " lithological similarities," prefer not 

 to use the term Archcean at all, but to employ local terms for 

 the oldest crystalline rocks exposed in a given district. 



The Archaean includes the most ancient rocks, often spoken of 

 as the " basement, or basal complex." Its antiquity is best as- 

 sured in regions where it is separated by thick series of sediment- 

 ary or metamorphic rocks from the Lower Cambrian, which can 

 be certainly identified by its fossils. In such regions the Archaean 

 is composed of completely crystalline rocks of various types con- 

 fusedly mixed together, massive rocks, such as granite and basic 

 eruptives, and foliated rocks, like gneissoid granite, gneiss, and 

 various schists, are intermingled in the most intricate way. Some 

 of these rocks cut the others in the form of dikes and are mani- 

 festly of very different dates of formation. The dike rocks may be 

 either massive or schistose. The component minerals are princi- 

 pally orthoclase and acid plagioclase, quartz, hornblende, and mica, 

 with other minerals as accessories. The particles show plainly the 



