244 PALAEOZOIC TIME — UPPER SILURIAN. 



that at this time the same coast-region must have become an ex- 

 tensive 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 shallow waters, emerging sand-flats, and 

 low shores. 



The clays, clayey sandstones, and limestones of the Clinton 

 epoch through New York and the Appalachians show that the 

 Medina condition of the coast-region still continued, except that 

 the marshes were at times shallow seas where 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 limestones, indi- 

 cate that the waters occupied 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, could not have been formed in an open sea ; 

 for clayey iron-deposits do not accumulate under such circum- 

 stances. They are proof of extensive marshes, and, therefore, of 

 land near the sea-level. The few 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 shales of the Niagara epoch on the east indicate no great 

 alteration of the coast-region after the Clinton epoch ; but the 

 increasing proportion of limestones to the westward, and their 

 great thickness and comparative purity in western New York, and 

 still greater prevalence in the States beyond, teach that the inte- 

 rior sea had become nearly what it was in the Trenton period. It 

 was, however, more beautiful in its life ; for Corals and Crinoids 

 were a marked feature of the epoch. 



Let us turn back now to the Trenton period, in review. There 

 was then a shallow sea over the whole interior (with small excep- 

 tions), and it covered even the Appalachians, for here, also, lime- 

 stones were formed ; and any sea-coast of sands, pebbles, or muddy 

 flats must have been farther east, where some portion of the con- 

 tinental border may have been raised to the surface through a 

 slight bending upward of that part of the earth's crust. (This 

 border now extends to a line 80 miles at sea [see p. 12 and fig. 664].) 

 In the Hudson period there appears to have been a partial sub- 

 sidence of the outer barrier and a shallowing of the interior sea by 



