STEATIGRAPHIC CLASSIFICATION STRUCTURAL CRITERIA 457 



to a layer less than a foot in thickness. Further, as suggested above, 

 this layer owes its existence chiefly to the fact that the overlap extends 

 to or across areas exposing older sandstone formations. For this reason 

 a thin basal sandstone is a rather common feature of formations devel- 

 oped on the flanks of Ozarkia, whose broad dome of Eopaleozoic rocks 

 comprises several important sandstone formations. On the Cincinnati 

 and Nashville domes, however, where old arenaceous deposits are almost 

 wanting, overlaps are but seldom indicated by quartzose initial deposits. 

 The principal, not to say the only, exception is the Chattanooga shale, 

 and this probably derived its basal sandstone from Ozarkia. 



As a rule, the percentage of argillaceous matter in limestone forma- 

 tions increases toward the shore, but even this change is generally not 

 very conspicuous. Indeed, it is with considerable surprise that we noted 

 the small thickness of impure reddish limestone that separates in every 

 way typical Pamelia limestone in New York and pure Lowville limestone 

 in the vicinity of Kingston, Ontario, from the pre-Cambrian floor. Sur- 

 prise was felt again when, after a long search near Theresa, New York, 

 the contact between the base of the Pamelia (upper Stones Kiver) and 

 the top of the Theresa (earliest Canadian) finally was located, and noth- 

 ing found, save an inch or two of fine conglomerate, to indicate that the 

 hiatus is of sufficient magnitude to represent deposition of 5,000 feet of 

 dolomite and limestone in central Pennsylvania. Also, again in New 

 York, when we found no conglomerate at all, but only a thin "passage 

 bed" at the base of an incomplete overlapping Lowville section to mark 

 the even greater hiatus at Middleville, where it rests on Saratogan (Little 

 Falls) dolomite, and at Little Falls, where the early Canadian Tribes 

 Hill limestone is beneath. And all this on the flanks of old lands that 

 at times have furnished large quantities of detrital material. After 

 such observations it seems quite proper to find new limestone formations 

 beginning with sediment no less pure than that above them. 



STRATIGRAPHIC HIATUSES 



Frequent absence of clastic material at base of overlapping formations 

 not extraordinary. — That in many good exposures scarcely a trace is to 

 be seen of the imperfectly stratified initial deposits referred to in pre- 

 ceding paragraphs will seem to many a strong argument against the 

 assumption in such cases of emergent conditions. In other words, they 

 may claim that in the absence of subaerially produced elastics, and per- 

 haps of other clear evidence of land decay and erosion, other possible 

 causes of stratigraphic hiatuses should be given preference. Willis^ ^ ad- 



ei Bailey WlUis : Science, N. S., vol. 31, No. 790, Feb. 18, 1910, p. 249. 

 XXXI — Boll. G ol. Soc. Am., Vol. 22, 1910. 



