Pensauken Formation — Description. 79 



to cobble beds with fine gravel and sand in the interstices, as at 

 Kingston. 



The g'ravel includes quartz, quartzite, sandstone, chert, shale, 

 crystalline rock such as granite, gneiss, schist, gabbro, diabase, 

 etc., and ironstone. No piece of limestone has even been seen 

 in it. Among the pebbles and bowlders, the amount of crystal- 

 line material present ranges from o to 10 per cent., shale from 

 o to 80, sandstone from o to 20, chert from 2 to 30, ironstone 

 from o to 70. The gravel may be scattered promiscuously 

 through the sand (Fig. 31), or it may be in beds or lenses (Fig. 

 36). It is, on the whole, more abundant at the bottom, and in 

 the upper part of the formation, than at intermediate horizons 

 (Figs. 37 and 38), but bodies of gravel in the middle portion are 

 by no means unknown (Fig. 3.9). 



The sand is quartzose, arkose, and in many places glauconitic. 

 Glauconite may be in any proportion up to 90 per cent., though 

 more than 10 per cent, is rare. Loam and clay are, as a rule, 

 present in small quantity only. 



Where arkose, the material is usually compact and coherent, 

 and is extensively used for road material. The abundance of 

 soft chert, shale, decayed bits of igneous and metamorphic rock, 

 together with decayed feldspar and loam, cause it to pack well 

 in road beds. In not a few places the material of the formation 

 is partially cemented (Fig. 40). Locally, faulting on a small 

 scale is shown, the gravel and sand being compact enough to 

 behave like solid rock during movement (Fig. 41, p. 104). 



Sources of material. — Among the stony materials of the Pen- 

 sauken the following can be identified : 



1 ) Schist like that which occurs at Trenton and Philadelphia, 

 is common in the Pensauken below Trenton, but cannot, as a 

 rule, be referred to any particular part of the schist area. 



2) Black shale, with abundant impressions of plants, like that 

 quarried at Milford, 30 miles above Trenton. Pieces of rock 

 of this sort have been seen most frequently in the Pensauken 

 formation between Burlington and Mount Holly, and about 

 Deacons Station. The shale is certainly from the Newark 

 series, though it may have come from; some point other than 



