CLAYS IN TERTIARY FORMATIONS. 139 



either rare or entirely wanting in them. As a whole, the flora 

 is more nearly comparable with that of certain European Upper 

 Miocene localities, and we may regard it as that which imme- 

 diately preceded the close of the Tertiary period, and this con- 

 clusion is emphasized by the well-recognized fact that in Europe 

 biologic evolution was in advance of America, so that the 

 European Eocene flora is largely comparable with the American 

 Miocene, European Miocene with American Pliocene, and 

 European Pliocene with the American living - flora." 



Mr. Hollick also states 1 that the leaf-bearing formation could 

 be referred either to the late Miocene or Pliocene on the basis 

 of these fossil plants. Some obscure casts of Molluscan shells 2 

 have been found near Millville, but they are not decisive. The 

 paleontological evidence, therefore, does not enable us to deter- 

 mine the age of the beds with any more certainty than does the 

 stratigraphy, although it suggests their Pliocene ag'e. 



Clay Deposits. 



Beds of clay, often of considerable extent, occur somewhat 

 widely in the Cohansey sand. They range in thickness up to 24 

 feet, as reported, but in the great majority of cases they are only 

 8 or 10 feet thick, rarely exceeding 12, so far as known. Hori- 

 zontally they vary from a few acres to several hundred in extent. 

 In only those cases in which the deposit has been exhausted, or 

 in which many borings have been made, is the size of these clay 

 lenses known. 



Distribution. — These clays occur in the southern portion of 

 the State, for the most part southeast of the divide separating the 

 tributaries of the Delaware from the streams which flow directly 

 into the Atlantic. That is, they occur in Ocean and Atlantic 

 counties, the southern portion of Burlington, Camden and Glou- 

 cester, and the central part of Cumberland. In other words, they 

 occur in the sandy pines district of the State. This is, in a general 

 way, the area southeast of the line on the map, Plate X, which 



Letter to the writers. 

 : N. J. Geol. Surv., Ann. Rep. for 1896, p. 254. 



