694 THE JOURNAL OF GEOLOGY. 



kie and the Keweenawan strata at various horizons, or in some 

 one of the many dykes cutting these. 



In the report 1 of the following year, upon referring to the posi- 

 tion of the gabbro with respect to the other formations, Prof. 

 Winchell says . . . "In general the gabbro lies on the 

 Animikie (Taconic) in Minnesota." At Chub (Akeley) lake, 

 however, it seems to be underlain by a bed of quartzite, regarded 

 as a lower member of the copper-bearing formation of the Pots- 

 dam (Keweenawan of Irving and Chamberlin) in the seventeenth 

 report, but looked upon as Animikie and denominated the 

 Pewabic quartzite in the sixteenth report, 2 and described under 

 the field name "muscovado" in earlier reports. 



In a more recent discussion 3 as to the age of the gabbro, 

 Prof. Winchell briefly summarizes his previous views on the sub- 

 ject, and concludes that the supposed quartzite underlying the 

 gabbro belongs near the bottom of the Animikie, and since the 

 eruptive rock is so closely associated with the fragmental one, 

 that the former must be of nearly the same age as the latter. 4 



This conclusion is based on the supposition that the rocks 

 immediately underlying the gabbro are fragmental quartzites that 

 have been altered by the eruptive for miles even from its contact 

 with them. 5 But this is probably not always the case. As the 

 writer 6 has shown in another place, some of the so-called quartz- 

 ites are very basic crystalline aggregates of pyroxene and oliv- 

 ine, and others are granulitic phases of the overlying gabbro. 

 Since they are portions of the gabbro they are of the same age 

 as this, and are not available as stratigraphical data for use in 

 determining the time relations of the great "flow" with respect 



1 17th Ann. Rept. for 1888. St. Paul, 1891, p. 52. 



2 i6th Ann. Rept, pp. 82-87. 



3 The Iron Ores of Minnesota. Bull. Minn. Geol. Survey, No. 6, 1891, p. 125. 



4 Cf. also: 20th Ann. Report, p. 2. 



s H. V. Winchell : lb. p. 127. 



6 Bayley W. S. : Notes on the Petrography and Geology of the Akeley Lake 

 Region in Northeastern Minnesota. 19th Ann. Rept. Minn. Survey. Minneapolis, 

 1892, p. 193 et seq. 



