GEOLOGY OF THE NORTHERN ADIRONDACK REGION 357 



the formation and are numerously exposed at various localities 

 on the north border of the Adirondacks in such situation as to 

 indicate clearly their horizon. At Potsdam itself the section 

 is complicated by faulting, and the horizon of the red sand- 

 stone there can not be demonstrated, though inferentially it 

 is low in the formation. Along with the red there is much hard, 

 glassy, brown sandstone, also containing fresh feldspars, but 

 lacking the hematite coloration of the red beds. Above, the 

 reds become striped and mottled with white, forming a species 

 of passage beds to the middle division. 



Van Ingen is the only observer who has undertaken to dif- 

 ferentiate between the middle and upper portions of the Pots- 

 dam. He says : 



The middle portion of the sandstone is made up of well sorted 

 materials, of finer grain, compactly cemented, and of white, steel 

 gray or yellowish color, with very little or no feldspatkic content. 

 The grains of sand are both angular and rounded, with the former 

 predominating. The layers are more regular, though their sur- 

 faces are ripple-marked, and in section they are seen to be almost 

 universally cross-bedded. Pebbles are found on the surfaces of 

 some layers of the middle portion, but unlike those of the upper 

 portion they seem to have been of soft mud derived by erosion of 

 contemporaneous sediments, cast on the beach at times of rough 

 water and flattened and squeezed out by the subsequent pressure 

 and consolidation of the superimposed sand deposits. 



The upper portion of the formation has frequent beds of 

 irregular laminated sandstone, with partings of greenish are- 

 naceous shale. The shale surfaces are covered with fucoids 

 and worm trails. Pebbles of shale and dolomite, which were 

 hardened before the time of their entombment, are found em- 

 bedded in the sandstone layers, and their disintegration causes 

 cavities to form in the layers containing them. The dolomite 

 pebbles become more abundant toward the upper horizons. In 

 the upper levels frequent beds are composed of nicely rounded 

 grains of clear quartz with a little cement, that crumble to a 

 sugary powder under the hammer. Kounded grains of quartz 

 of a slightly larger size occasionally cover the upper surface 

 of a layer of finer grained sandstone, and, being without cement, 

 they stand out in relief above the surface with an appearance of 

 having been sprinkled from a pepper pot. 1 



'Op. cit. p.543-44. 



