444 E. O. ULRTCH REVISION OF THE PALEOZOIC SYSTEMS 



shale formations. Possibly the lower Cambrian deposits thought to 

 belong in a fourth trough in fact preceded the marbles in the third 

 basin. However, judging from data now available, it has seemed pref- 

 erable to assign them to distinct troughs. As a possible compromise 

 between the two views, it may be suggested that the lower Cambrian 

 deposits are confined to the supposed fourth trough, but that the much 

 younger marbles, limestones, and shales are common to both the third 

 and fourth troughs. Eegarding the age of the Vermont white marble 

 formation, it must be confessed that the evidence is not positive. By 

 inference, based on stratigraphic position and probable movements con- 

 trolling location and character of deposits, the age of the marble is pro- 

 visionally determined as middle Canadian — that is, as approximately cor- 

 responding to Division D of Brainard and Seely's Champlain Valley "Cal- 

 ciferous." The limestone overlying the marble formation contains some 

 fossils, but their age relations are not exactly determined. There is 

 little doubt, however, that this overlying limestone represents an age 

 following the close of the Stones Eiver and preceding the Trenton, hence 

 as correlating with some part or parts of the Blount and Black Eiver 

 groups. The shale formation which closes the sedimentary record in 

 this trough is referred with considerable confidence to the Martinsburg 

 shale, a formation that, like the correlatives of the underlying limestone 

 and marble formations, is widely distributed in and to the east of the 

 Appalachian Valley proper. The fine-grained white and pink marbles 

 which outcrop at intervals from beneath overthrust formations to the 

 east of the valley from A^ermont to Alabama seem all to be, according 

 to present evidence, essentially of the same (middle Canadian) age. At 

 any rate, I have met with no evidence positively opposed to this view. 



As already intimated, the sediments in the supposed fourth trough are 

 the quartzites, shales, and limestones of lower Cambrian age now found 

 in Washington County and other parts of New York east of the Hudson. 

 Similar deposits containing a comparable if not the same Olenellus fauna 

 occur in the mountainous eastern rim of the Appalachian Valley from 

 northern New Jersey to Alabama. The strikingly uniform character of 

 these deposits long ago suggested the now generally accepted view that 

 in early Cambrian times the sea invaded the inland surface of eastern 

 America only along a narrow down-warped band, stretching southward 

 from Canada to central or southern Alabama. The land to the east of 

 this trough probably was not very high, but was covered with a deep, 

 well decomposed regolith (rock mantle), much of which was swept into 

 the deepening inland trough. It is doubtful whether at this time the 

 submergence of the trough extended down the Saint Lawrence to New- 



