44 THE GEOLOGY OF MINNESOTA. 



[Base of the Taconio. 



that there are weighty considerations, based on the chemical characteristics of 

 potassium, its comparative specific gravity and its avidity for oxygen, that tend to 

 indicate that potassium remained in the earth's atmosphere not only longer than the 

 other alkalies or alkaline earths, but also during that long heated condition of the 

 earliest crust occupied by the slow congealing, and finally the cooling of the first 

 formed film before it was possible for the ocean to rest upon it. 



The view above presented of the origin of the igneous rocks, especially those of 

 the Archean, is essentially the same as that adopted by Button (1785), Keferstein 

 (1834), Herschell (1836), Hunt (1858), Le Conte (1872), Button (1880) and numerous 

 other geologists. But these geologists labored under the wrong idea that the oldest 

 rock was essentially granitic, and much of their reasoning was vitiated by an attempt 

 to explain the rock succession from the alkaline to the ferro-magnesian instead of 



the reverse. 



THE TACONIC. 



The detrital rock formations included under this term in Minnesota are separated 

 from the Archean by a profound break in the stratification, as well as by difference 

 in crystalline condition. They are divided between Animikie and Keweenawan, and 

 are supposed to be the representative of the Lower and Middle Cambrian. 



The base of the Taconic. The conglomerate which occurs at the base of the 

 formation has been examined at various places in Minnesota, viz., at the south side 

 of the Giant's range (No. 372H), S. W. i N. W. J sec. 32, T. 60-13, where it lies upon 

 the granite of the Giant's range and embraces much debris from the granite. It is 

 exposed to the depth of twelve or fifteen feet. At the top it contains considerable 

 green uncrystallized material, with large feldspar fragments, and has an indistinct 

 gneissic structure which seems to lie nearly horizontal, with a schistosity running 

 northeast and southwest. At the top are some pebbles of quartz and green rock, 

 some of them two inches long. In the field this conglomerate seems to grade into 

 the granite.* A diamond drill section was made in N. W. J S. W. J sec. 27, T. 60-13, 

 passing from black slates through quartzyte, ore, quartzyte, and conglomerate, 

 into the Giant's range granite, the whole depth being 323 feet. The base of the 

 Animikie here contained not only feldspars from the granite, and lavender-blue 

 quartz grains from the same source, but also much gi'een debris from the adjoining 

 and underlying Keewatin. The coarsely conglomeratic portion of the Animikie here 

 was only three or four feet in thickness. In some other drill sections the conglom- 

 erate was also quite thin, and in some instances the basal quartzyte was found to lie 

 directly upon the granite with no apparent conglomerate. A thin conglomeratic 



"Owing to the fact that the supposed Aniinikie is immediately in contact with granite on S. E. ,' 4 N. E. ;,;, sec. 35, T. 61-12, 

 and shows no conglomerate, as well as on account of the nature of the ore and of the conglomerate itself at the above locality, there 

 is some uncertainty as to the Animikie age of the conglomerate. The granite may here be intrusive into the Upper Keewatin, and 

 into the Lower Keewatin at other points in the vicinty. See Seventeenth Report, pp. 86, 94. 



