TAGONIC QUESTION IN GEOLOGY. 133 



the thickness of this series of Cambrian sandstones, shales, conglomerates and limestones, 

 but says that it " is manifestly very great in eastern New York." '" 



§ 156. It is hardly necessary to mention that this series of Cambrian fossiliferous rocks, 

 traced by Ford through Eensselaer, Columbia and part of Duchess Counties, along the 

 eastern side of a belt of Loraiue shales, is a part of the great Grraywacke belt, the age of 

 which was dis^juted between Emmons and Mather, (the Hudson River group of the latter), 

 and which Logan, after his examination of the region with Hall, in 1863, described and 

 subsequently map]ped as Quebec group. These observers, as has been already stated 

 (§ 115), and as may be seen on Logan's map of 1866, then traced a narrow but persistent belt 

 of Loraine shales along the eastern side of the Hudson, from Washington County south- 

 ward to a point a little above Hyde Park, where they found the boundary between these 

 shales and the older group to cross to the west side of the Hudson. The accuracy of this 

 delineation is confirmed by Ford, who, while remarking that the distribution of the upper 

 rocks might entitle them to be called the Hu.dson Hiver group, suggests, in view of the 

 perplexities which have attended its use, that it would be better " to discard altogether 

 the designation, and go back to the old term, Loraine shales." Ford farther speaks of the 

 " great dislocation," which, at so many points from western Vermont to the Hudson in 

 Duchess County, brings up the Cambrian rocks against newer strata of Ordovician age. 

 A reference to the sections of Logan and Billings, already cited, will, however, show the 

 existence, not of a single dislocation, but of parallel dislocations, with upthrows on the 

 east side, towards the barrier of older rocks. Of such parallel faults we find, in fact, re- 

 peated examples, not only east of the Hudson, but farther southward, along the eastern 

 border of the Appalachian valley, as already shown in § 101. 



§ 15*7. The one continuous break, with an upthrow on the south and east of *7,000 

 feet, extending from G-aspé to Alabama, imagined by Logan, was required in his struc- 

 tural scheme, because he had assumed the Levis limestone, (which near Quebec is brought 

 to adjoin the Loraine shales,) to occiipy a position at the base of his Quebec group, and to 

 have been originally buried *7,000 feet beneath the Loraine shales in a great con- 

 formable series. The strata along the west side of these dislocations in Canada and in 

 Vermont are, according to Logan, either Levis, Chazy, Ti-enton or Loraine, the Lower 

 Potsdam being on the east side. In a section described by Billings, and already noticed 

 (§ 148), where the first dislocation brings up the Lower Potsdam — which is successively 

 overlaid by Calciferous, Levis, Chazy and Trenton — against the Loraine, a second parallel 

 fault, a little farther to the east, brings up the Levis against the Trenton. We see, from the 

 late studies of Ford, that the great belt along the eastern border of the Loraine shales, 

 which Logan described and mapi^ed as Quebec group, is in large part Lower Potsdam. 

 The whole series must now be farther studied in the present light : we must know the 

 real thickness of the Cambrian in the region in question ; the interval therein which 

 separates the Lower Potsdam from the Levis fauna ; and how much of the Quebec group 

 of Logan is to be included in the Potsdam. 



§ 158. As regards the relations of the Cambrian and Ordovician rocks over this area, 

 we have already shown that there is every reason to believe that there exists a stratigraph- 

 ical break between them, (as is also the case between the Lower Taconic and Cambrian), 



'- Amer. Jour. Science, 1884, xxviii, pp. 35 and 206. 



