Page:The Journal of geology (IA journalofgeology11893univers).pdf/268

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 position 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 Potsdam (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 subject, 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 quartzites are very basic crystalline aggregates of pyroxene and olivine, 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