258 PROCEEDINGS OF THE ACADEMY OP [1884. 



The extreme dryness of this marl, compared with the preceding 

 varieties, is owing to the complete absence of argillaceous matter. 

 The grains, when examined, are smooth and well rounded, and 

 although frequently so soft as to be easily crushed by the nail, 

 show no evidence of chemical decomposition. 



Shell Layer. — This intermediate, well-defined layer, is best 

 exposed at the head of Noxontown mill-pond, and along the south 

 side of Appoquinimink Creek. In thickness it varies from 3 to 10 

 feet, being entirely made up of white friable shells, tightly packed 

 together, the most common of which are Terehratula fragalis and 

 T. Harlani^i with Pychnodonta vesicularis. 



Very often the upper part of the shell layer has lost its carbo- 

 nate of lime, which is replaced by brown oxide of iron. At the 

 head of Noxontown milLpond, the white shell layer, of which the 

 upper part is ferruginous, rises 5 to 6 feet out of the water, 

 capped by a yellow sand marl. 



Yellow Sand. — This is the uppermost laj-er of the Middle 

 Marl Bed, and is alwaj^s found associated with and overlying the 

 shells. It may be described as a yellowish or reddish sand, con- 

 taining a small and variable proportion of glauconite, the latter 

 often becoming so predominant as to give the deposit a decidedly 

 greenish tinge. 



DiORiTic Trap. 



Three miles to the south of Newark, Delaware, Iron Hill rises 

 from the Cretaceous plane, the one conspicuous object for 10 

 miles or more. Running in a generally northwest and southeast 

 direction, it has a length of over 3 miles, a width varying from 

 1 mile to 1^, and a height of 225 feet. The flanks and summits 

 of this hill are covered with boulders of diorite and cellular 

 quartz. On the south side, about half way up, is seen the 

 outcrop of a bedded mass of serpentine rook, with a strike 

 following the trend of the hill. An examination of the loose frag- 

 ments of gTcen rock lying upon the surface showed them to be 

 composed of a number of indefinite chloritic and serpentine 

 materials in a state of partial decomposition. As the greenish 

 fragments were also observed to run into unchanged trap, 

 occurring as huge outcropping boulders, the proof appeared 

 conclus^ive that the serpentine rock had been due to the alteration 

 of the hornblende in the dioritic trap. Following the hill in its 

 northwest course, we find the same rock crossing the railroad, 

 along the cutting of which the nature of the dike is revealed. 



