Page:The Journal of geology (IA journalofgeology11893univ).pdf/325
Minnesota as resting unconformably upon a pre-Paleozoic floor. The base of the Potsdam is usually conglomeratic. At Minnecopa a well 800 feet deep passed through a conglomerate for a distance of 225 feet, the pebbles of which are vitreous quartzite like those occurring in Cortland, Watonwan and Cottonwood counties. A conglomerate containing granitic debris is found on Snake river about two miles above Mora, and three miles distant from the Ann River knobs of hornblende-biotite-granite, the clastic appearing to be derived from the granite. At Taylors' Falls a conglomerate made up of pebbles of diabase rests upon the diabase of the St. Croix river. These underlying formations are Archean and Algonkian rocks.
Grant[1] states that the Animikie rests unconformably upon the Saganaga granite; that the Ogishki conglomerate is intruded by the Saganaga granite, and therefore that the Ogishki conglomerate is earlier than and separated by a great structural break from the Saganaga granite. As the Keewatin has the same relations to the Saganaga granite as the Ogishki conglomerate, the same thing is true of the Animikie and Keewatin. The Ogishki conglomerate is younger than the most of the Keewatin, but is considered as a part of it.
Comments:—The most characteristic and abundant fragments in the Ogishki conglomerate are granite. The rock occurs in pieces running from those of minute size to great bowlders. It is manifest that this material was derived from a pre-existing granite. These bowlders are in all respects like much of the Saganaga granite, and the probability is very strong that this is their source. Grant is probably correct as to the intrusion of the conglomerate by a granite, but this granite may have also intruded the main Saganaga mass. The too frequent mistake has apparently been made of concluding that in the Saganaga area there is granite of but one age, when frequently in the great massives of the Northwest, granites of several ages occur, the latest ones cutting all the previous ones, and often the far newer clastics. It further follows that the implication that the Animikie is unconformably upon the Ogishki conglomerate needs the support of additional evidence. That this conglomerate is possibly more nearly related to the Animikie than to the Keewatin is shown by the presence of abundant jasper fragments, presumably derived from the Keewatin. The article appears to be another illustration of the facts being right, the author in his interpretation, however, overlooking a part of the facts which are to be accounted for in making a true generalization.
Winchell[2] gives a review of the literature on the Norian of the Northwest. Here are included the gabbros, placed as the basement member of the