236 PALEOZOIC TIME. 



and probably opened on the ocean to the southeast. The existence of 

 such interior evaporating flats implies intermittent incursions of the 

 sea, perhaps only through tidal overflows, but also, probably, such oc- 

 casional floodings as may take place where coast-barriers or reefs are 

 broken through at times by the waves or currents. 



As the Saliferous beds of New York are nearly 1,000 feet thick, just 

 west of the centre of the State, and since there is proof in the shrink- 

 age-cracks and other peculiarities that the layers were successively 

 formed in shallow waters, it follows that there must have been a slow 

 subsidence of the region during the progress of the period, — it may 

 have been of but a few inches or feet in a century. 



3. LOWER HELDERBERG PERIOD (7). 

 I. Rocks : kinds and distribution. 



The Lower Helderberg period was marked by the formation of thick 

 limestone strata. There was a gradual passage to its clear open seas 

 over New York, from the great sea-marshes of the Salina period. The 

 period is so named because its beds are well displayed in the Helder- 

 berg Mountains, south of Albany, beneath DeA r onian beds called the 

 " Upper Helderberg." 



The lower beds are designated the Water-lime group ; they overlie 

 directly the Salina beds, in New York, and aj>pear as if a continu- 

 ation of them. Moreover, they spread through the State, from the 

 Hudson River to its western border, while the rest of the series does 

 not reach west beyond Ontario County. The whole thickness in 

 eastern New York is 400 feet. A single isolated summit of Lower 

 Helderberg rocks, called Becraft's Mountain, stands just east of the 

 Hudson River, near the city of Hudson ; and another is Mount Bob, 

 three miles to the northeast : these are evidently remnants of a great 

 formation that once spread widely in that direction. Another isolated 

 patch occurs near Montreal. 



The Helderberg rocks outcrop also over a large area in western 

 Ohio, and are continued thence into Indiana. They come out to view 

 also in southern Illinois. 



South of New York, along the Appalachian region, they extend 

 through New Jersey, Pennsylvania, Maryland, and Virginia, increas- 

 ing in thickness, being in all 500 feet or more on the Potomac ; and, 

 as in the North, they diminish westward. 



The subdivisions of the formation observed in the Helderberg 

 Mountains are for the most part undistinguishable out of New York 

 State. The lowest rock, the Water-lime, retains its characters most 

 widely, and has a thickness of 350 feet on the Potomac (Rogers). The 



