222 THE GEOLOGY OP MINNESOTA. 



[Conglomerate. 



rock No. 143. It is the same great diabase which began first to be seen at Silver 

 creek, and which is traceable to this place, and much further. For a detailed repre- 

 sentation of the stratigraphy at Baptism river see Part III. N. H. w. 



No. 149. CONGLOMERATE, (fail.) 



About one-half mile below the first falls of Baptism river. This rock appears on the east bank, dipping, 

 with a synclinal bend, 10 south, by 10 east. It is isolated from all other outcrops and the dip of all strata 

 seen furtner up the river would cause this to overlie them if there be no other irregularity. Runs twelve rods 

 along the shore, the greatest exposure being eighteen feet; varies to a red sandstone, but the greater portion is 

 full of pebbles. Some of the pebbles are six inches across, but generally they are smaller. 



Eef. Annual Report, ix, page 38; Annual Report, xiii, pages 100 (No. 160), 103. 



Meg. In addition to the field description, this rock may be said to be made up 

 essentially of volcanic materials, evidently derived from the Cabotian eruptives of the 

 region. The larger pebbles are of apobsidian and volcanic debris of the same, and 

 the finer of the same and of softer elements. It is largely cemented by calcitc. It 

 has the appearance of the conglomerate at the Calumet and Hecla copper mine on 

 Keweenaw point, but lacks, so far as known, metallic copper and quartz-porphyry. 

 It is also somewhat less firm than that. 



Mic. The composition of this conglomerate is almost exclusively of devitrified 

 glass (with calcite cement), some of it being almost entirely of glass still. The 

 pebbles show scattered small spherulitic growths, and a few indeterminable microliths, 

 yet some of them are themselves composed of an earlier volcanic grit or tuff, 

 in which the constituents are finer and angular, and embrace both quartz and 

 plagioclase. 



Two sections. 



Cliemical analysis. An analysis of this rock gave the following results: 



Si0 2 66.72 



A1 2 0, 7.41 



Fe 2 3 10.13 



FeO .69 



CaO 3.10 



MgO 4.06 



K 2 O -42 



Na 2 O -86 



H,0 - 5.32 



Total 98.71 



Aye. Puckwunge; supposed base of the Potsdam. 



Remark. This conglomerate is believed to be somewhat lower than that seen 

 at Two Harbor bay, on the west (No. 817), but nearly the same as that seen again 

 toward the east at the lake shore (Nos. 155 and 155A), and to be included in the base 

 of the later division of the Keweeuawan for this region, the red pebbles which it 

 contains being derived from the Cabotian volcanic and red-rock series. N. H. w. 



