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 "Athens Marble," 

 so extensively employed in Chicago, is quarried from this 

 formation. It much resembles the famous " Kentucky Mar- 

 ble," 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 OA r er 

 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, 



