THE GLACIAL DRIFT. 629 



and over. The several kinds of red porphyry erratics are of very 

 doubtful origin. No such rock occurs in the Huronian or Lauren- 

 tian of North Wisconsin or Michigan, nor am I aware that any oc- 

 curs in the Copper series of Lake Superior, except in the state of 

 conglomerate pebbles, which have evidently been derived from an 

 older series. The limestone pebbles of the drift have come from all 

 the Silurian limestones of eastern Wisconsin, the Galena and Niag- 

 ara formations having furnished the larger part. These formations 

 extend in continuous belts from the south to the north line of the 

 state, so that it is not often possible to say in what direction the peb- 

 bles have come. 



The origin of the sand and clay of the drift may be considered 

 in the same connection, though not affording more than a general 

 idea as to the direction of the drift movement. The great prepon- 

 derance of sand over the other drift materials, in much of Central 

 Wisconsin, is without doubt to be attributed to the great surface 

 spread of the friable Potsdam sandstone in the region over which the 

 drift has passed. Sand is, however, also found forming most of the 

 drift even far north in the Archaean district, where it is sometimes in 

 quantity sufficient to produce sand barrens. This fact may be regarded 

 as proving a much greater surface extent north and east in the 

 Archaean area of the sandstone formation in preglacial times. The 

 clay has come partly, perhaps, from the limestone formations, and 

 partly from the kaolinization of felspathic erratics, but its principal 

 source would seem to have been the previously kaolinized granites 

 and gneisses of the Archaean region. It is well known that in all 

 southern regions where the drift phenomena are unknown, as for in- 

 stance along the Blue Ridge from Virginia to Alabama, and in Bra- 

 zil, the felspathic crystalline rocks are found rotted to great depths. 

 Hunt has drawn attention to the fact that in the region of the Blue 

 Ridcje this ceases to be the case north of the southern limit of the 



O v 



glacial drift, whose deposits lie upon the hard, unaltered, and often 

 polished rock surface, and has inferred the removal of the softened 

 rock by the glacial forces. In that small portion of the Archaean 

 region of Wisconsin, in which the drift is insignificant or wanting 

 entirely as along the valley of the Wisconsin south of Stevens 

 Point, arid along Black river south of the crossing of the Green Bay 

 road decomposed and kaolinized gneiss and granite occur. Over 

 the rest of the Archaean region, on the contrary, the drift rests directly 

 upon the unchanged rock. 



No fossils of any kind have ever come to my attention as occurring 

 in the drift deposits of Central Wisconsin. 



