STRATIGRAPHIC RELATIONS OF MONMOUTH FORMATION. 333 



micaceous sandy clays. This may find its explanation in either one of 

 two waj^s. It may be the result of actual unconformity or it may be 

 brought about by the change of the sandy members of the upper Mon- 

 mouth in northern New Jersey, until they gradually become replaced 

 by clays toward the south, to be succeeded by sands upon the eastern 

 shore of Maryland, where the formation again thickens. The structural 

 relations are not sufficiently clearly defined to absolutely determine this 

 point, although there are strong indications in certain places, shown in 

 the marked change in the general aspect of the materials and the sharp 

 lines of contact, to indicate that unconformity exists. In that case it 

 is not impossible that the sands of the upper Monmouth have suffered 

 removal through central and southern New Jersey, although this would 

 not be necessary, since both the thinning out of the sands and gradual 

 replacement by clays, as well as unconformity, could occur, and the ex- 

 planation of the relations observed may be found in a combination of the 

 two hypotheses. The unconformity, if it exists, doubtless represents a 

 very brief interval, since the general conditions did not largely change, 

 while the life forms of the previous age persisted in a number of instances 

 into the later period. Although most careful and detailed observations 

 and measurements have been made in the area where the sands finally 

 disappear, there is still some question as to the presence of unconformity ; 

 yet it may safely be assumed as a tentative hypothesis, with the reserva- 

 tion that it may wholly or in part be accounted for by the marked change 

 in the character of sedimentation. 



Divisions — General characteristics. — The Monmouth formation can be 

 subdivided upon lithologic grounds throughout a large part of its area 

 into three members, which are especially well marked in the northern 

 portion of the district and upon the eastern shore of Maryland, but are 

 less clearly defined in central and southern Now Jersey and upon the 

 western shore of Maryland. The three divisions in ascending order are 

 the Mount Laurel sands, the Navesink marls, and the Redbank sands. 

 The latter two have been previously named from typical areas in Mon- 

 mouth county, while the first receives its designation from mount Lau- 

 rel, situated to the east of Camden, in Burlington county, where the 

 sands of this horizon are most extensively developed. 



Mount Laurel sands. — These sands are, on the w r hole, perhaps the 

 most constant member of the Monmouth formation, although very vari- 

 able in thickness and changing considerabty in their character as they 

 extend southward into Maryland. They consist typically of coarse red 

 sands that are often indurated on account of the large amount of iron 

 present in them. They are more or less glauconitic, especially toward 

 the south, and in their more unweathered portions, when reached by 



