GEOLOGICAL EXPLORATIONS AND LITERATritE— 1S91. 123 



followed in turn by tbe accnniulatiou in the usual horizontal position of the irou- 

 beariuji' iletrital series, whose folding and erosion were still later iirocesses. And yet 

 this folding and erosion all preceded the deposition of the horizontal Cambrian sand- 

 stones of the region. (Pp. 2.')-24.) 



A ('(intiriiiatit)U <it" these conclusions is furnished, according to Irving, 

 bv the similarity between the great dikes, cutting the greenstone-schists 

 and tlie sheets of eruptive greenstone in the iron-ljearing series. 



For the latter greenstones are in large measure interatratifled with the sedi- 

 mentary layers of the iron-bearing series, following the bending of its layers; so that, 

 even if these greenstones are in the nature of intruded sheets, it seems necessary to 

 believe that their intrusion took place before the folding of the iron-bearing series. 

 , Now, tbe corresponding dikes in the greenstone-schist area were evidently intruded 

 subsequent to the production of the scliistosity of the intruded rocks. If, then, 

 these are facts, the time when the iron bearing series was folded was very much 

 subsequent to that time at wliieh tbe greenstone schists received their schistosity. 

 (P. 24.) 



The nia}) acconipanA'iug Irving's paper is jiracticallv Roniinger's map 

 interpreted according to Irving's ^'ie^v. That is, the greenstone-schists are 

 placed with the granites l)elow tlieiron-ljearing series. In the legend of 

 the map we see for tlie first time the use of the term Algoidvian, by which 

 it was decided b^• the United States Geological Survev to designate the 

 fragmental series lying Ijetween the Archean crystallines and the base of 

 the Cambrian. The term stands in the period place as equivalent to the 

 Agnotozoic group. 



Williams, G. II. The greenstone-schist areas of the -^lenominee and Marquette 

 regions of Michigan UnW. U. S. (Jeol. Survey No. (i2, Washington, 1891, pages 

 134-217. With plates of thin sections. 



Williams's paper deals primarily with the microscojjical features of the 

 green schists already so frequently mentioned. 



The author divides the district studied into four areas: (1) The Easteiii 

 area, near Marquette; (2) the Western area, immediately north of Teal 

 Lake, in the town of Negaunee; (3) the Northern area, lying north of 

 Dead River; and (4) the Deer Lake area. The Eastern area is further 

 divided into a northern and a southern half 



