598 RECORD OF MEETINGS OF THE 



found at the base of the New Scotland beds, where the latter 

 are in contact with the Longwood and shales. The source of 

 the iron is evidently from the red shales, but whether the con- 

 tact was due to overlap or faulting has not been previously 

 explained. Two thirds of a mile north of the mine the Decker 

 Ferry, Cobleskill, Rondout, Manlius and Coeymans formations, 

 having a total thickness of ninety- five feet, are found between 

 the New Scotland and Longwood beds. In the region of the 

 mine the strata are nearly vertical, and in faulting a wedge- 

 shaped block has been forced up, bringing the red shales in 

 contact with the New Scotland beds. A cap of limestone has 

 until recent geologic times protected from erosion the mass of 

 soft Longwood shales, which now form a steep hill that is rapidly 

 being worn away. 



In discussing types of sedimentary overlap, Dr. Grabau 

 said that with a normal sea-shore a rising sea-level will pro- 

 duce the phenomenon of progressive overlap, a falling sea-level 

 that of regressive overlap. If the sea transgresses slowly, and 

 the rate of supply of detritus is uniform, a basal rudyte or arenyte 

 is formed which rises in the column as the sea advances, and 

 whose depositional off-shore equivalents are successive beds- 

 of lutytes or organic deposits (biogenics). Types of such 

 basal beds which pass diagonally across the time scale are 

 seen in the basal Cambric arenytes of eastern North America, 

 which as the Vermont Quartzyte are Lower Cambric, and as 

 the Potsdam are Upper Cambric. Again in the basal Cretacic 

 arenyte of southwestern United States, this is shown, they 

 being basal Trinity in Texas, Washita in Kansas, and Dakota 

 or later on the Front Range. Examples of this type of pro- 

 gressive overlap are numerous and familiar. On an ancient 

 pemeplain surface the transgressing sea may spread a basal 

 black shale, as in the case of the Eureka (Noel) Black shale, 

 which is basal Choteau in southern Missouri and basal Burlington 

 in northern Arkansas. Regressive movements of the shore 

 succeeded by transgressive movements give us arenytes which 

 are enclosed in off-shore sediments and which within them- 

 selves comprise an hiatus the magnitude of which diminishes 

 progressively away from the shore. An example of this has 



