GEOLOGigAL EXPLOKATIONS AND LITERATURE— 1885. 101 



Irving, R. 1). Prelimiuary paper on an investigation of tlie Arcliean forma- 

 tious of tbe northwestern States. Fifth Ann. Kept. U. S. Geol. Snrvey, for 1883-84, 

 "Washington, 1885, pages 175-2413. With maps. 



Ill this paper we find for tlie first time a definite statement of the 

 j^roblems to be attacked in ^vorkillo■ out tlie structure »t' the Lake Superior 

 region, and a report of the author's success in solving- tliem. 



The first problem which he attacks is the stratigraphic relation of tlie 

 bedded rocks. "In the purelv granitic areas this qiu^stion will not present 

 itself, and it is very doubtful whether anything in the way of a determination 

 of a succession of layers can be accomplished in the regions where the 

 rock is inainh' gneissic." In some of the Iluroiiiaii areas the succession of 

 layers can easil}- he worked out, but in the Marquette area, "besides the 

 obstacle of frequent heavy drift covering, an additional and more serious 

 difiiculty is met with in the complex folding to which the rock layers have 

 been subjected " 



The second problem which presents itself fia- solution is the structitral 

 relations of the iilainly sedimentary rocks to others that are, or mi\y be, of 

 eruptive origin, such as the greenstones of the district )irovisionally called 

 Huronian. 



The third prolilem is the origin of the gneisses and granites, of the 

 schists associated with these rocks, and of the iron ores and jaspers among 

 the Huronian rocks. 



The fourth problem is one of correlation between the rocks of difierent 

 portions of the lake region. This does not concern us at present. 



With respect to the Manjuette series, the author writes (pp. ls;i-l!X)): 



The rocks of this series are highly fokled, and their structure is often very 

 difficult to work out. Moreover, the luetasomatic changes which the crystalline 

 members of the series have undergone have often been extreme; added to which 

 diflticulties are frequent interruptions by drift covering. 



Although studied more minutely and by more different authorities than any 

 other portions of the region, the divergencies of view as to structure and genesis in 

 this region, even among the later writers, have been very great. • * * 



After having obtained a thorough acquaintance with the rocks of this region 

 and those of the type region of Lake Huron, no doubt remains in my mind as to the 



