ONWARD THROUGH THE AGES. 125 



shore of Lake Michigan to Chicago, and even to Joliet, 

 when it bends westward and northwestward, and loses 

 itself beneath the accumulations of a later period. The 

 quarries at Lockport, New York, and many others in that 

 vicinity, are located in this important limestone. In the 

 same formation are those at Milwaukee, Waukesha, Chi 

 cago, Lamont, and Joliet. The so-called &quot;Athens Marble,&quot; 

 so extensively employed in Chicago, is quarried from this 

 formation. It much resembles the famous &quot; Kentucky Mar 

 ble,&quot; from which the beautiful monument and statue to 

 Henry Clay, at Lexington, is built though the latter comes 

 from the Trenton group, in the Lower Silurian. 



The second period was that of the Salina group, which 

 has become famous for the production of salt and gypsum, 

 in the vicinity of Syracuse, New York. Its outcropping 

 belt runs in a line parallel with that of the Niagara lime 

 stone throughout its whole course, as far as Milwaukee. I 

 shall hereafter offer some explanation of the circumstances 

 under which salt and gypsum have accumulated to such 

 an enormous extent in certain formations. 



The third period was that of the Lower Helderberg 

 group, which is not found to be generally spread out over 

 the country like the other two. In New York it is espe 

 cially developed in the Helderberg Mountains, where Pro 

 fessor Hall has obtained a rich harvest of organic remains. 

 It was here that he found the type of that magnificent 

 crinoid, which he so beautifully named Mariacrinus, in com 

 memoration of the assistance and sympathy of his accom 

 plished wife in his life-long scientific labors. It thins out 

 and disappears in Western New York. This group is known 

 again in Southern Illinois, where it has been brought to 

 light by the indefatigable and well-directed labors of Pro 

 fessor Worthen ; and in Missouri, where it has been illus 

 trated by Swallow and Shumard; and, finally, in Maine, 



