156 GEOLOGY. 



they best adjust themselves. The constitution of the system makes 

 it clear that it does not represent the original crust of the earth or its 

 downward extension. It cannot be affirmed, however, that no part 

 of what is now classed as Archean is referable to the original crust; 

 that is, it cannot be affirmed that no part of the igneous or meta-igneous 

 rock of the Archean is referable to an azoic or pre-zoic period, strong 

 as the evidence against such reference may seem. On the other hand, 

 all the facts now known concerning the Archean adjust themselves 

 to the planetesimal hypothesis, or to the modified form of the gaseo- 

 molten hypothesis. They cannot, however, be said to establish either, 

 or to preclude other hypotheses of the origin of the earth. The question 

 of the origin of the Archean must, therefore, still be regarded as an 

 open one. According to the planetesimal hypothesis, the Archean 

 is looked upon as representing the time of transition from the earlier 

 Formative eon to the later Gradational eon (p. 119). According to 

 the modified form of the gaseo-molten hypothesis, it represents the 

 formations of the volcanic era which followed the formation of the 

 original crust. 



Earlier Views Concerning the Archean. 



In explanation of the Archean system, many different hypotheses 

 have been suggested at one time and another, most of them starting 

 with the Laplacian hypothesis as a beginning. Some of these hypothe- 

 ses have at least a historic interest. One is that the Archean rocks are 

 completely metamorphosed sediments; a second, that they are igneous 

 rocks produced by the fusion of sediments ; and a third, that they are 

 igneous rocks intruded beneath the oldest known sedimentary rocks 

 after the deposition of the latter. 



1. The hypothesis that the Archean rocks are metamorphosed 

 sediments was originally based on the assumption that schistosity and 

 gneissic structure were an expression of stratification; that is, that 

 everything which resembles stratification is really such. So long as 

 this view obtained, it was natural that the gneisses and schists should 

 be regarded as of sedimentary origin; but since it is now known that 

 such structures may be developed in sedimentary rocks, independent 

 of stratification, and in igneous rocks as well, the argument loses its 

 force. 



If the Archean rocks are primarily metamorphosed sediments, it 



