CLAYS OF CRETACEOUS FORMATION* 163 



such definite subdivision would be possible in the case of the 

 Raritan as has been made in the Clay Marls or Marl series. 



Thickness. — The thickness of the Raritan series varies con- 

 siderably, as is shown by numerous well borings which have 

 penetrated to the older rocks beneath. Cook and Smock made an 

 estimate of 347 feet from a detailed study and comparison of the 

 various beds in Middlesex county. Our own estimates for this 

 region give 380 to 390 feet. At Asbury Park, 20 miles down 

 the dip from the outcrop, a well penetrated 367 feet of beds 

 "belonging to this series without reaching its base; whereas, at 

 Bordentown, the entire thickness of the formation, from top to 

 bottom, as shown by borings, does not exceed 250 feet. At 

 Jobstown, southeast of Bordentown and 8 miles farther down the 

 dip, the Raritan was penetrated for 409 feet, and bottom not 

 reached. At Delair, north of Camden, the base of the Raritan 

 is 162 below tide. Making due allowance for the upper part of 

 the formation not present here, owing to post-Cretaceous erosion, 

 the thickness is 275 to 300 feet. The greatest thickness, how- 

 ever, has been reached in a boring at Fort Dupont, Del. (oppo- 

 site Fort Mott, N. J.), where 594 feet of the strata belonging 

 to the Raritan were penetrated without reaching its base. These 

 figures indicate that the formation is thinner along its outcrop 

 than down the dip to the southeast. This conclusion is in accord 

 with what is known of other formations of the coastal plain. 



Stratigraphic relations. — As has already been stated, the con- 

 tact of the Raritan and the Clay Marls above is sharp and easily 

 recognized. Wherever seen, its top is a loose sand, or a sand 

 with clay laminse, whereas the base of the Clay Marl is a glau- 

 conitic clay, black when fresh, a rusty brown where weathered, 

 and frequently fossiliferous. The contact is, moreover, fre- 

 quently emphasized by a bed of ironstone due to' cementation of 

 the upper layer of sand. 



The basal contact is less frequently shown. In Middlesex 

 county, wherever exposed, the Raritan beds rest unconformably 

 upon the eroded edges of the Triassic (Newark) shale, while 

 some well borings show that locally they are underlain by trap 

 rock. From Trenton, southwestward, they rest upon the Phila- 

 delphia gneiss and schist, as is shown by a few well borings. So 



