No. 136.] 39 



not seen as a distinct or separable mass in the Mohawk valley, 

 though the rocks above and below it are there well known. In 

 the far West, in Iowa, the place of this rock in the series is occu- 

 pied by what is called the St. Peters sandstone, there 60 or 80 

 feet in thickness ; and this sandstone forms the lower part of the 

 precipice at the Falls of St. Anthony, being there covered by 

 the Trenton limestone, the lower part of which forms the brink 

 of the fall and the floor of the rapids above. . Southward this 

 limestone (together with the succeeding Black-river limestone) 

 becomes enormously developed by the thickening of its parts and n 



the addition of other strata; so that, if w^e may rely on the 

 measurements of the Pennsylvania geologists, it becomes in that 

 state from 2500 to 5500 feet in thickness! * Like most of the 

 rocks of that region, it is there not very prolific of fossil re- 

 mains. 



The Chazy limestone contains a considerable variety of fossils, 

 among which the most conspicuous is the Maclurea magna (Fig. 2), 

 a remarkable coiled shell ; in which the coil, though close and 

 having a nearly flat surface on the top, is open and forms a deep 

 central hollow below. Some layers of this limestone, which are 

 worked extensively as a dark gray marble for hall pavements, 

 frequently show white spiral coils, which are merely sections of 

 this shell, split through the middle by the saw of the stone- 

 cutter. Such specimens are to be seen in the halls of the Delavan 

 House in Albany. 



There are also in this rock several kinds of bivalve shells 

 belonging to the great class of " Brachiopoda ;" the name of 

 which, derived from the Greek brachys, an arm, and pons, a foot, 

 refers to a very peculiar internal arrangement of these molluscs, 

 which consists mainly in the presence within the shells of two 

 spiral coils or arms, which bear " cilia " or minute vibratory 

 filaments, by the motion of which the animals are believed to 

 create currents in the water to enable them to gather their food. i . 



Mollusca of this class are rare among living shells, but were 

 enormously abundant in the earlier periods of earth's history. 

 They are arranged in many genera, among which are Spirjfer, ^: 



in which the shell is straight in the hinge-line, and often pro- j 



* Many of the groups of strata, as has been heretofore mentioned, show similar variations 

 in thickness in different parts of their extent; the sediment from which they were formed 

 appearing to have been deposited in great thickness at some points, wbil© at others it waa 

 very thin or entirely wanting. 



