CLAYS IN TERTIARY FORMATIONS. 141 



The upper foot or two of the clay not infrequently contains 

 scattered pebbles, and the whole is usually covered by several 

 feet of sand or gravel or both. Locally, the change to the over- 

 lying sand or gravel is abrupt and sharply marked, and the 

 upper limit of the clay is apparently an erosion surface on which 

 the sand and gravel were later deposited (Plate XVII, Fig. 2). 

 When this is the case, the upper part of the clay is without 

 stones. Elsewhere, there is, for a. space of a few inches or a foot 

 or two, an alternation of sediments, laminae of clay being con- 

 tained in the overlying beds. In a few instances these clay laminae 

 continue for several feet above the main mass of the clay. 



There seems little reason for doubting that the clay and the 

 underlying sand were successive deposits, formed without inter- 

 ruption. In those cases, where the clay is somewhat sharply 

 separated from the overlying sand and gravel, the former is 

 best referred to the Cohansey, and the latter to the Bridgeton or 

 Pensauken, or regarded as a secondary deposit derived from 

 them by weathering and washing. Where the clay is apparently 

 commingled in its upper part with the overlying gravel, the latter 

 may still be a much later deposit, the commingling of the two 

 being due to re-working of the upper portion of the clay as the 

 gravel was deposited. In this case the bulk of the clay might 

 be Cohansey, with its upper portion re-worked in Bridgeton or 

 Pensauken time. In a few rare instances the clay, by its inter- 

 lamination with the overlying sand and gravel, seems to be of 

 the same age as the gravel, rather than older. Although in the 

 light of our present knowledge it seems best to regard all these 

 clay lenses as belonging in the Cohansey sand, yet the possibility 

 that some of them may be later in age (perhaps Bridgeton) must 

 be kept in mind. 



THE SHILOH MARE. 



A gray, highly fossiliferous marl is found in a limited area in 

 Salem and Cumberland counties along the tributaries of Stow 

 creek. In the region in which it occurs this formation next under- 

 lies the clay-bearing Cohansey sands. Since it contains fossils 

 of undoubted Miocene age, the overlying sands and clays cannot 



