REVIEWS 109 



quartzite, which locally contains iron enough to constitute a low-grade 

 ore. From this horizon a considerable amount of iron ore was for- 

 merly obtained. The lowest member of the series, in a number of 

 instances, was found to pass by almost imperceptible gradations into 

 the underlying granitoid gneiss, in such a manner as to suggest the 

 decidedly rapid submergence of a deeply weathered Cambrian land 

 mass, with a correspondingly rapid advance of the sea over the same, 

 affording insutScient time either for the thorough sorting of the loose 

 materials already at hand or the bringing in of any considerable 

 amount of sediment from a distance. The entire basal series, repre- 

 senting, as it does, distinctly littoral or at least shallow-water deposits, 

 has a total thickness of only a few hundred feet at the most, and the 

 conditions under which it was deposited must have rapidly changed 

 to those necessary for the deposition of the off-shore and distinctly 

 deep-water sediments represented by the two or three thousand feet 

 of dolomites and the dolomitic limestones, which immediately suc- 

 ceed it. 



The series is the northeastern extension of beds which in York 

 county have been called by Walcott "the Hallam quartzite'" and is 

 the equivalent of the Hardiston quartzite of Kiimmel and Weller"" in 

 northern New Jersey. 



Post- Glacial Time. By A. H. Elftman. 



Hitherto the St. Anthony gorge has been ascribed to the St. 

 Anthony falls, which is regarded as having decreased in height from 

 Fort Snelling to Minneapolis. Several features of the gorge were 

 described, showing that it was formed largely by rapids. The falls 

 did not assume prominence until two miles above Fort Snelling was 

 reached, and they have been increasing in height. Account is taken 

 of the terraces in the Mississippi valley, and the differential uplifts are 

 recognized as affecting this region. The gorge represents a much 

 longer period of time than has been assigned to it. This is further 

 strengthened by the evidence afforded in the postglacial gorge of the 

 St. Croix river. 



The Relation between the Keezvatin a7id Laurentide Ice-sheets. By 



A. H. P2LFTMAN. 



Evidence was presented to show that the glacial drift of the upper 

 Mississippi river valley was deposited by independent lobes of the 

 ^BulL U. S. Geol. Surv., No. 134, 1897. 

 'Bull. Geol. Soc. Am., Vol. XII, pp. 149 ff. 



