558 HISTORICAL GEOLOGY. 



canthus (Claypole). The telson in Fig. 797 is half too short ; it was partly buried in the 

 rock when drawn, and has been recently uncovered by C. E. Beecher. 



The Tentaculite limestone has afforded Camarocrimis stellatus H. (a form found also 

 in Bohemia), Stropheodonta varistriata Con., Spirifer Vanuxemi, Tellinomya nudei- 

 formis H., Modiolopsis (?) duhia H., Avicula obscura H., Holopea subconica H., 

 H. antiqua H., H. elougata H., MurcMsonia extenuata H., M. minuta H., Oncoceras 

 ovoides H., Cyrtoceras subrectuni H., Spirorbis laxus H., BeyricMa trisulcata H. 



3. The Lower Helderberg Period. 



rocks — kinds and distribution. 



The preceding Onondaga formation has been described as extending far 

 eastward, as well as westward, but as having its greatest thickness in central 

 New York, central Pennsylvania, and Ohio. The Helderberg beds not only 

 extend far eastward, but, in contrast with the preceding, have their greatest 

 thickness to the eastward, and thin out in western New York. They are 

 doubtfully recognized in Ohio, 20 feet being the greatest thickness reported. 

 The representative rocks over the Central Interior Sea have not been 

 made out. 



East of Hudson River the beds constitute the low, isolated elevation 

 called Becrafts Mountain, near Hudson, excepting its basal layer (the 

 Water-lime) and the upper stratum, which is of the Oriskany sandstone 

 with the Cauda-galli grit ; also the smaller and similar Mount Bob, not far 

 distant to the north. Each of these hills is all that is left of a great for- 

 mation after ages of denudation. Logan was probably right in his conclusion 

 that it once extended northward, along the Hudson River and Lake Champlain 

 valleys, to Montreal ; for similar beds occur on the island of St. Helens in 

 the St. Lawrence, opposite Montreal, resting on Utica shale of the Lower 

 Silurian. Hence the waters of the Eastern Interior Sea during this Lower 

 Helderberg era had resumed their deep connection with the waters of the 

 St. Lawrence region about Montreal. 



The beds are 300 to 400 feet thick in eastern New York, 350 feet in 

 central Pennsylvania (Perry County), and 600 in eastern (in Monroe County), 

 and in New Jersey. They occur also in the Appalachians in Virginia, but not 

 in eastern Tennessee. They are 20 to 100 feet thick in western Tennessee, 

 and 175 feet thick in Missouri, but are not distinct in Illinois or Wisconsin. 

 In other words, the beds are either thin or wanting over the Central Interior 

 region. 



The St. Lawrence tidal waters of this period must have extended westward 

 to the borders of Vermont and Montreal and southward along the Connecticut 

 valley. In Canada, in the line of the Connecticut valley. Lower Helderberg 

 fossils occur in Dudswell and near Lakes Massawipi and Aylmer. They are 

 also found in northern New Brunswick, northern Maine, near Square Lake, 

 and along the Gaspe- Worcester trough. They also occur in southern New 

 Brunswick and near the coast in Pembroke, Me., with many fossils, and in 

 northern Nova Scotia, within the limits of the Acadian trough. 



