196 WALKS AND TALKS. 



erally the upper half. Like the Potsdam Sandstone it is 

 huffish in color, and disposed to crumble to pieces. In the 

 north-west it is known as the Lower Magnesian Limestone. It 

 contains the lead mines of Missouri (Talk XXI). Above this 

 comes the St. Peters Sandstone, white, clean, and destitute 

 of fossils ; but this is not known at the east. Next is the 

 Trenton Group, which contains the great Trenton Limestone. 

 Like the other great central limestone masses (Niagara, Cor- 

 niferous, Lower Carboniferous) this forms a conspicuous land- 

 mark across the country, and constitutes the rich repository of 

 the remains of the animals which dwelt in the Upper Cam- 

 brian ocean. This limestone mass forms the bluffs at St. Paul 

 and Minneapolis; comes up on the north side of the Mani- 

 toulin Islands ; stretches westward across Ste. Mary's river, and 

 running through the Upper Peninsula of Michigan, goes down 

 along the west side of Green Bay, into southern Wisconsin, 

 northern Illinois, and north-eastern Iowa, holding the lead 

 mines in these three states ; outcrops over a large area about 

 Cincinnati, extending to Madison and Richmond, Indiana, and 

 Frankfort and Lexington, Kentucky ; outcrops again at Nash- 

 ville and surrounding region ; stretches through central New 

 York to Watertown, and across the St. Lawrence to Georgian 

 Bay, stretching along its eastern shore and emerging again at 

 the Manitoulin Islands. Everywhere, this group of limestones 

 and shaly limestones is wonderfully rich in the remains of 

 creatures which swarmed in the seas of the twilight ages of the 

 world. Most delicate structures, most exquisitely preserved, 

 which never cease to excite our admiration and our wonder. 

 Many a geologist has devoted a large part of his lifetime to 

 their study, and it seems a pity that we must pass them by 

 with a simple mention. 



