THE ARCHEOZOIC ERA. 



145 



have been crushed and sheared so as to develop a foliated or schistose 

 structure. 1 It is in the rocks of this era primarily that metamorphism 

 of this type is most commonly found. This is now regarded as having 

 been a very extensive phenomenon, the larger part of existing gneisses, 



Fig. 37.— Figure showing the crumpled and contorted structure characteristic of 

 much of the Archean rock. From a Canadian locality. (Ells.) 



as well as a considerable portion of existing schists, having acquired 

 their foliated structure by mashing and shearing. It is not to be over- 

 looked, however, that some of the schists and perhaps some of the 

 gneisses arose from clastic formations by anamorphic processes. 



Distribution and Local Development. 



General distribution. — The Archean is the one accessible rock 

 system which is theoretically universal, in the sense that it underlies 



1 E. Emmons, Report of the Midland Counties of N. C, 1856; Lieber, Second 

 Ann. Rept. on the Progress of the Surv. of S. C, 1858; Irving, Geol. of Wis., Vol. I., 

 pp. 340-61; Adams, F. D., Rept. of the Brit. Assoc, for the Adv. of Sci., 1886; Wil- 

 liams (G. H.) Bull. 62, U. S. Geol. Surv. and Proc. Am. Assoc, for the Adv. of Sci., 

 36th meeting, 1887; and Van Hise, Sixteenth Ann. Rept., U. S. Geol. Surv., Pt. I. 

 and others. 



