STRUCTURAL GEOLOGY. 51 



The Puckwunge conglomerate.] 



from the iron-bearing member of the Animikie (Nos. 852B and 852C). It is first seen 

 in Minnesota in the valley of the Puckwunge, a small stream entering Pigeon river 

 about a mile south of South Fowl lake (Nos. 1903 and 2069). The thickness here 

 exposed, including the associated grit, amounts to 144 feet, with probability of as 

 much more below the visible portion. 



The dip is toward the S. W. by S. 12, and is distinct. The rook is essentially a white quartz pebbly 

 conglomerate, the coarsest stones being about six inches in diameter, rounded lenticular and hard, altogether 

 water-worn. There is very rarely a distinct banded red-jasper pebble, and some that are not banded, and more 

 common a gray, siliceous pebble like basanyte. Some of the pebbles are reddish brown. The great majority of 

 them are of vein quartz, but some appear of chalcedonic fineness of grain. The general character and appear 

 ance of the mass are like those of the quartz-pebble conglomerate seen in the St. Louis valley, a short distance 

 above Fond du Lac. 



In collecting pebbles (No. 2069) a special effort was made to obtain some that might prove to be charac- 

 teristic of some earlier formation, and among these the rock taconyte was sought for. Thin sections made from 

 some of these showed that the rock from which they were derived was of clastic structure, though now composed 

 of secondary silica and hematite and identified with the taconyte of the Animikie, thus fixing the conglomerate 

 later than the Animikie. One of the most common sorts of pebble is that of a gray, granular quartzyte, which is 

 easily referable to the slaty quartzyte of the upper part of the Animikie.* 



This conglomerate is known again at one mile southwest from Grand Portage village, described by Mr. 

 Elftman as follows: It is in a high bluff facing the lake, appearing white as viewed with a glass from Grand 

 Portage island. The rock making up this bluff is coarse, white quartzyte, becoming conglomeratic in places. 

 Thin pebbles of slate are scattered throughout the rock. It is apparent that only the upper part of the formation 

 is exposed here. Quite a distance intervenes between the conglomerate and the siliceous member of the 

 Animikie, in which there are no outcrops. The exposed thickness is about 100 feet. The stratification of the 

 sandstone is very plain. There is a diabase flow twenty feet thick, superposed on the conglomeratic quartzyte, 

 fine grained and compact in its lower portion, and amygdaloidal in its upper. Small portions of the quartzyte 

 are included in the diabase, which resembles that found on Grand Portage island. Above this diabase is a 

 stratum of quartzyte varying from a few inches to two feet in thickness; and above this are successive layers of 

 diabase with more or less of detrital matter between them. The successive layers of diabase, and all the quartzyte, 

 are cut by a dike of diabase about vertical, running east and west, about ten feet in width. The general strike 

 of the conglomerate seems to connect it with that seen in the Puckwunge valley and at Grand Portage island. 

 The distinctions between the Animikie siliceous slate and the conglomerate or quartzyte are as follows: 



Animikie quartzyte. Slaty, dull or earthy appearance, very fine grained, cut by Keweenawan dikes; also 

 by post-Keweenawan dikes. 



The Keweenawan conglomerate contains flat pebbles of the Animikie slate and quartzyte. It is usually 

 fresh and bright in appearance. It is cut only by the later dikes. On Grand Portage island it also contains 

 red rock pebbles. 



The structural conditions are expressed by the following diagram (figure 1), 

 drawn by Mr. Elftman. In this diagram the supposed place of the Grand Portage 

 graywacke is in the interval of " no outcrop." 



FIG. 1. CROSS SECTION AT GRAND PORTAGE. 



The first point in the region of Grand Portage at which this conglomerate was 

 observed was on Grand Portage island (No. 254), where it shows about twenty feet of 



*Compare Twenty-fourth Annual Report, pp. 34-39, for the details of the stratigraphic relations of this conglomerate in 

 the Puckwunge valley. 



