UPPER SILURIAN. 231 



under the action of strong marine currents. The making of the Green 

 Mountains must have placed the region more or less above the sea- 

 level ; for the absence of the earlier formations of the Niagara period 

 from eastern New York, and the thinning eastward of the Niagara 

 beds, harmonize with this view. 



The fine sandy and clayey character of the Medina beds shows that 

 at this time central New York must have become an extensive area of 

 low, sandy sea-shores, flats, and marshes, not feeling the heavy waves ; 

 and this kind of surface extended westward over Michigan, instead of 

 having a limit in central New York. There is abundant evidence, in 

 the ripple-marks, wave-marks, rill-marks, and sun-cracks, of the exist- 

 ence of shallow waters and emerging sand-flats. 



The clays, clayey sandstones, and limestones of the Clinton epoch, 

 through New York and the Appalachians, show that the mud-flats 

 and sand-banks, and hence the shallow seas of the coast region, still 

 continued, yet with some greater depth of water at times, in which 

 impure limestones could be formed ; and the many alternations of 

 these limestones with shales and sandstones imply frequent changes 

 of depth over these areas, as remarked by Hall. At the same time, 

 the westward extension of the formation, and the prevalence of lime- 

 stones, indicate that the waters covered a considerable part of the 

 Interior Continental basin ; while the impurity of the rock suggests 

 that these inner seas were in general quite shallow. The beds of 

 argillaceous iron -ore, which spread so widely through New York and 

 some of the other States west and south, could not have been formed 

 in an open sea ; for clayey iron-deposits do not accumulate under 

 such circumstances. They are proof of extensive marshes, and, there- 

 fore, of land near the sea-level. The fragments of Crinoids and shells 

 found in these beds are evidence that they were, in part at least, salt- 

 water marshes, and that the tides sometimes reached them. 



The beds of the Niagara epoch on the east indicate that the waters 

 shallowed toward the Hudson River ; at the same time, the thick 

 limestones of western New York and the Mississippi basin teach 

 that there was then a great open interior sea, nearly as in the 

 Trenton period, though more beautiful, since Corals and Crinoids 

 were a more prominent feature of the era. 



If the above is a correct view of the geographical changes, it is seen 

 that, after the Medio-Silurian revolution, which raised the Green 

 Mountain region, even eastern New York was, in the first two epochs 

 of the Niagara period, above water ; but there was then a gradual sink- 

 ing of the land, which moved the coast-line in New York eastward to 

 the Hudson, so that, over New York and the Interior basin, there 

 was a vast limestone-making sea. We infer that this oscillation of 



