312 THE STRUCTURE AND SUCCESSION AT 



In all this section, sand beds are replaced laterally by shales, 

 or thin out into them. The clay must have accumulated until 

 the water was comparatively shallow, then sand spread over it, 

 thinning out as depth increased. After this had gone on for 

 some time, the sea bottom was depressed and into the deepening 

 water only mud could be brought. The clay overspreading the 

 sand and extending beyond it enclosed a wedge of sand similar 

 in shape and situation to those now exposed. 



Main seam. — This seam is not well exposed in the cliti', so 

 that information concerning it had to be gained from the mines. 

 It is six feet thick, overlain by soft flaggy gray shale contain- 

 ing particularly good plant remains. The following are especi- 

 ally abundant : — -Neuropteris cordata, Cyclopteris acadiana, 

 Pecopteris, Alethopteris, and leaves of Cordaites. 



Details of section to Cranherry Head. — Fifty feet above 

 the Main seam lies a limestone horizon two and one-half feet 

 thick, and crystalline. Three feet of marl and shale separate it 

 from two inches of black bituminous shale, containing a multi- 

 tude of fish scales and shells of Naiadites. Twelve feet above 

 is a second similar layer, in which the shells compose nearly 

 the whole mass. The species represented are Naiadites elong- 

 atus and N. laevis. They are supposed to be proof of brackish 

 water or lagoon conditions. 



The shell beds are succeeded by shale with a little sand- 

 stone to a depth of fifty feet, which in turn is followed by 

 four inches of coal with a calcareous underclay. Three feet 

 above the coal lies one foot of limestone. The underclay could 

 not have got its lime from washings from this bed, for in that 

 case, the intervening shale would also be calcareous. This is 

 not the case ; consequently the underclay must have accumulated 

 its lime independent of the bed above. 



From this level to the top of the visible layers at Cranberry 

 Head, clay ironstone nodules become more abundant. In some 

 beds, they make up nearly one-fourth of the whole material. 



The strata at the particular horizon now reached are less 



