158 GEOLOGY. 



intruded beneath the sedimentaries after the deposition of the latter, 

 and that they were brought into their present lithological condition 

 by metamorphism subsequent to their irruption. This hypothesis 

 is distinct from the last in that it involves the intrusion of the igneous 

 rock, rather than its formation in situ, and in that it does not specify 

 that the source of the lavas was sedimentary rock. It is like the pre- 

 ceding in that it makes the Archean a meta-igneous series. 1 Since 

 the sedimentary rocks overlying the Archean do not, as a rule, show 

 the contact metamorphism which this hypothesis demands, and since 

 the unconformity between the two series is generally an unconformity 

 of erosion, not of irruption, the hypothesis fails to explain the facts. 

 Furthermore, the hypothesis, as a general explanation of the Archean, 

 is an audacious one, for it supposes that igneous intrusions have come 

 in at the base of the known sedimentary series, not in one place only, 

 but generally where the base is accessible, thus commonly concealing 

 the floor on which the oldest sediments were deposited. So violent 

 an assumption makes the hypothesis based upon it improbable. 



Though the hypothesis can hardly be accepted as a general explana- 

 tion of the Archean, it does not follow that the oldest sedimentary 

 rocks nowhere rest, with irruptive unconformity, on igneous rocks. 



The Archean in other countries. — The general characters and 

 relations of the Archean complex already given for North America 

 seem to be duplicated in other continents. Corresponding systems of 

 rocks, made up primarily of meta-igneous, but subordinately of inex- 

 tricably involved meta-sedimentary rocks, are known in all continents. 

 The general characteristics and relations of the Archean, as developed 

 in North America, therefore appear to be essentially world-wide. 



In Great Britain, the Lewisian gneiss appears to correspond with 

 the Archean of North America. This formation was derived by me- 

 chanical deformation from a complex aggregate of eruptive rocks of 

 different ages, and in one area there appears to be a group of still more 

 ancient sedimentary rocks through which the gneiss was intruded. 2 



In Norway, coarse-banded gneisses (Grundfjeldet, Urberget), with 

 a wider petrographic range than the Lewisian gneiss of Great Britain, 

 are regarded as Archean. The gneisses seem to have originated from 

 acidic and basic eruptive rocks. With the gneiss is associated another 



1 For review of this hypothesis, see Van Hise, Bull. 86, U. S. Geol. Surv., p. 480. 



2 Geikie's Text-book of Geology, 4th ed., Vol. II, p. 888. 



