162 The Petrography and Genesis of Sediments 



which broke up the action of the water. The lowest part of the bed might 

 therefore be found slightly coarser on analysis, but no such difference 

 was noted in the field. 



It is to be noted that while this bed rests directly on Potomac there 

 must have been a preceding Upper Cretaceous transition over the region 

 (to have furnished the glauconite which in this bed is reworked), fol- 

 lowed again by a period of erosion which cut down to the Potomac beds. 



Minor features to be especially noted in this sample are : 



(1) The well-rounded grains of quartz in the coarser sizes. 



(2) The strongly marked solution surface on most of the coarser 

 grains. 



(3) The secondary quartz grains. 



(4) The grains representing a transition stage between clay and glau- 

 conite. 



(5) The abundant black mineral granules in the glauconite and in the 

 micas. 



SAMPLE NO. 12 (PIG. L, p. 169) 

 Serial number : 3. 

 Field number : 11-9-28-1911. 

 Formation : Monmouth. 

 Locality : Sassafras River. 

 Appearance : Loose, coarse, gravelly, dark greenish-brown sand with crumbly lumps of 



sand in a matrix of grayish-white clay. The loose sand appears to be mainly 



rounded grains of yellow-stained quartz. 



Mechanical Analysis 



Sample 9.235 gm. 



Per cent of 

 sample 



Fine gravel 4.2 



Sand 71.1 



Clay 25.2 



Total 100.5 



Per cent of 

 total sands 



Coarse sand 25.2 



Medium sand 49.4 



Fine sand 16.2 



Very fine sand 6.0 



Extra fine sand 3.0 



Total 99. 8 



Per cent of 

 very fine sand 



Light 14.0 



r ("Rejected at 2000 ohms 1.8 



av>- ^ Attracted at 2000 ohms (glauconite) 74.5 



Total 90.3 * 



1 It was at first not intended to weigh the products of this separation. Thus they 

 were not weighed till after microscopic study when some had been lost. They are 

 given to show the great dominance of glauconite (the portion attracted at 2000 ohmsK 



