54 NEW YORK STATE MUSEUM 
studied and hence need not ibe considered here. The total thickness 
of Siltric strata along the line of outcrop in the State varies coa- 
siderably. In central and western New York the thickness is gen- 
erally from 1000 to 1500 feet, while in southeastern New York it 
is much less since only the thinned upper formations are present. 
The first Siluric sediments to form in central and western New 
York are called the Oneida conglomerate and Medina sandstone, 
these two being of practically the same age. These coarse deposits 
were washed into the shallow sea from the northern lands, that is in 
Canada and the Adirondack region. Next im order came the deposits 
of Clinton age, which consist of layers of shale, sandstone and iron 
ore. Above the Clinton come the Lockport and Guelph formations 
made up of shales and dolomitic limestone, the limestone forming the 
crest of Niagara Falls. None of the formations, so far mentioned, 
extend to the Hudson valley, but with the opening of the great Salina 
epoch Siluric deposits for the first time reached to the Hudson valtey 
region where the earliest rock to form was the Shawangunk con- 
glomerate which rests upon the eroded Ordovicic shales at the sum- 
mit of Shawangunk mountain. This rock is entirely confined to 
southeastern New York, while rocks of the same age in central and 
western New York are shales and limestones. In this latter region 
the lowermost (oldest) Salina formation is the Vernon red shale, 
usually from 100 to 300 feet thick, which, in the western part of 
the State, is overlain by the salt and gypsum beds. Deep wells have 
proved the presence of the salt beds under practically all the South- 
western plateau. The salt was deposited in great salt lagoons and 
the climate of the time must have been arid. With an influx of 
fresh water into the lagoons, the type of deposit changed, and the 
hydraulic limestone (water lime) beds were formed all the way 
across the State to the Hudson valley region. These water lime 
beds are quarried at many places along the line of outcrop across 
the State, but more especially in the famous Rosendale cement 
region (see plate 33). Next in order, and marking the summit of 
the Siluric, come the Cobleskill, Rondout, and Manlius limestones 
which, though not very thick, are remarkably persistent across the 
State. 
As to the life of the Siluric seas it may be said that it is in effect 
the continued existence of the same organic groups that preceded 
in the waters of Ordovicic time, though some diminished, some in- 
creased and some new ones made their first appearance. Thus the 
graptolites and trilobites greatly diminished, while the echinoderms 
(star fishes) increased, and the brachiopods and mollusks held 
