548 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



before resubmergence set in. Such a break, providing the base of the 

 overlying formation is the oldest deposit of its age known, establishes 

 further that this withdrawal, and the emergent condition following it, 

 occurred during a distinct interformational period, the depositional 

 record of which, if not wholly confined to the oceanic basins, is at least 

 completely buried beyond present accessibility. If the top bed of the 

 underlying formation is not its youngest, then the question whether the 

 first deposits above the break are or are not equivalent to strata else- 

 where referred to the top of the underlying formation may remain un- 

 answered. If fossil evidence on the point is inconclusive principles 15, 

 17, and 18 may help in deciding the question. Similarly the solution of 

 the problem remains incomplete when a formation at its maximum is 

 succeeded by other than the oldest beds of an overlapping formation. 

 The following examples will illustrate the principles and bring out the 

 differences between the three conditions alluded to. It is to be observed 

 that the principle applies in essentially the same manner in establishing 

 extensive emergence between stratigraphic units, whether their rank be 

 that of a system, series, group, or formation. The first example throws 

 very interesting light on the time value of the new Ozarkian system and 

 on its stratigraphic relations to the Cambrian. 



The Ozarkian illustration. — The Ozarkian system as developed in the 

 southern part of the Appalachian Valley comprises the various magnesian 

 limestone formations which succeed the last of the upper Cambrian 

 shales and thin limestones, and precede the Canadian limestone or, where 

 that is absent, the Stones River limestone of the Ordovician system. 

 The whole of this sequence of dolomites and magnesian limestones is 

 commonly referred to a single great formation — the Knox dolomite. 

 However, this composite Knox varies greatly from place to place in its 

 thickness and in the age of the beds contained in it. In Knox county, 

 Tennessee, where the formation was first studied and from which it was 

 named, the Knox consists mainly of a characteristically and profusely 

 cherty middle division to which I am applying the name Copper Eidge 

 chert. This is flanked above and beneath by much thinner and very 

 sparingly cherty members, for which no names have been proposed. This 

 being the typical Knox, and as all the other beds commonly referred to 

 the Knox are readily distinguishable, it seems advisable to confine the 

 term to it. At its maximum, as observed in Hancock Count}^, Tennessee, 

 a few miles south of Sneedville, the typical Knox attains a thickness of 

 about 3,400 feet. At other localities, however, the upper and lower mem- 

 bers are thicker than here, so that the aggregate maximum of these divis-. 

 ions is considerably greater. (See Part III, pages 633 to 640.) 



