chert and limestone, and on the other into a rock presenting the 

 appearance of Tripoli : interstratified with these are beds of lime- 

 stone, occasionally oolitic. This group is not rich in organic re- 

 mains ; Crinoidete, Polypi/erce and Product ce are most common. The 

 middle and lower beds of this group are regarded by Dr. Owen as 

 probable equivalents of the upper Ludlow rocks. 



We next in descending order arrive at a group of bituminous, 

 aluminous shales and associate limestones, the lowest of which 

 affords a valuable water cement. In the shale there are no fossils 

 except a few slight impressions, apparently of seeds or seed-vessels. 

 Where the shale is replaced by indurated clay. Dr. Troost has found 

 Encrinites and Polypifera, and the " encrinital limestone " over the 

 shale in Tennessee is rich in Crinoidea. Atrypa prisca, Orthis lu- 

 nata vel orbicularis, Terehra sinuosa, Calymene bufo, and Asaphus ma- 

 crurus occur in the water limestone. The shale, Dr. Owen considers, 

 must probably be referred, as well as the water limestone, to the 

 lower Ludlow, and* may be regarded as the equivalent of the Hel- 

 derberg group and Marcellus shales of the New York geologists. 

 The encrinital limestone and the green ferruginoxis rock of Indiana 

 may correspond with the Aymestry limestones. 



Next in order is a group consisting almost wholly of compact 

 limestones, lying in thick beds without any interstratified marls or 

 shales. This rock is best developed toM'ards the north-west, and in 

 certain districts becomes a true magnesian limestone upwards of 

 500 feet in thickness. It closely approximates, both in lithological 

 character, mineral contents, and even proximity to the coal mea- 

 sures, the " scar limestone " of England, and were it not for the or- 

 ganic remains might be mistaken for it. But, says Dr. Owen, the 

 list of organic remains supplies proof hardly contestable, that the 

 rocks in which they occur are equivalents of the Wenlock formation 

 of Murchison. In the upper beds, Catenipora escharoides and Pen- 

 tame^'us huspodus are very abundant, with numerous other species 

 recorded by the author in his memoir. In the lower hundred feet 

 of this group fossils are scarce. Rich and important lead mines 

 occur in it, the most valuable in the United States. The most cha- 

 racteristic fossil of the lead-bearing strata is the Coscinopora. 



Next in order follow thin beds of shell limestone, alternating with 

 marl and marlite, occupying a superficial area of about 10,000 square 

 miles. The thickness of this group is greatest about the centre of the 

 Ohio valley, where it is 'estimated at 1000 feet. In the north-west, 

 at Prairie du Chien, it is but ] 00 feet, and near the Blue Mounds in 

 Wisconsin, but a few feet in thickness ; it abounds in organic re- 

 mains. Among these are characteristic, Isotelus gigas, Triarthrus 

 Bechii, several si^ecies of Conotubularia, and of Bellerophon and 

 Maclurites ; Isotelus planus, Lingula Lewisii, Orthis excentrica, 

 Orthis alata, and Asterias antiqua. These fundamental rocks of the 

 Ohio valley Dr. Owen considers the equivalents of the lower Silurian. 



No inferior rocks are visible in a north-west direction until the 

 vicinity of the Wisconsin river, where the blue fossiliferous lime- 

 stone rests conformably on a sandstone succeeded by a magnesian 



B 2 



