FEATURES OF TRAP EXTRUSIONS IN NEW JERSEY 305 



THE SHALES 



These are well described by the term "ferruginous sihcilutytes " 

 of Professor Grabau's classification; that is, they are finely-com- 

 minuted siliceous material, strongly impregnated with oxide of iron. 

 Their laminations may be paper-like in thinness but are generally 

 coarser. On exposure to the weather they break up into a multitude 

 of crumbly fragments. Mica scales are very plentiful. The surfaces 

 of the laminae frequently show a multitude of irregular markings — 

 grooves, pits, curved lines, lumps, smooth patches of irregular shape, 

 etc., not all of which can be deciphered with any certainty. Many, 

 however, can be identified. Mud-cracks, rain-pits, and worm-grooves 

 are frequent. Rill-marks are sometimes found. At times films of 

 impalpable sediment are found in the depressions in the lumpy surfaces 

 of certain sandstone layers, which, in their delicate markings, suggest 

 irresistibly the frothy scum left in hollows after a rain. Again thin 

 curled layers of shale appear plentifully in the midst of sandstone. 

 The effect is precisely that which would be produced by a layer of 

 mud, drying and curling at the edges, being buried by a deposit of 

 wind-blown sand. The layers of mud are too fragile to permit the 

 supposition that the sand was carried by currents of water. Other 

 examples show plentiful flakes of shale mixed with sand, the occur- 

 rence being apparently due to a mixture of material produced by the 

 wind. I have seen wave-marks in the shales and have found them 

 also in the finer sandstones. In the latter too I have found one very 

 distinct print of an apparently three-toed reptile. The impression 

 is a trifle over an inch in width and on one side is an irregular, curved 

 groove, probably made by the animal's tail. In the fine shales there 

 are many impressions which suggest the marks which would be made 

 by living creatures moving about in soft mud. They are too uncertain, 

 however, to offer more than a suggestion. In other localities, as in 

 the Connecticut valley and other parts of New Jersey, many reptile 

 trails have been found, but those spoken of here were found in the 

 territory under discussion in material taken from a short distance 

 below the trap sheet, and serve as an index of conditions just pre- 

 ceding the flow. 



These shales are to be regarded as muds deposited in lakes, rivers, 

 or pools. The markings described owe their preservation to the 



