254 PHYSIOGRAPHY AND GEOLOGY OF THE COASTAL PLAIX PROVINCE. 



The Jamestoivn Portland Cement Corporation s property is located at 

 Yorktown on the south side of York Kiver. The marl beds are exposed 

 in bluffs ranging from a few feet up to 40 and 50 feet along the York 

 River, and in ravines which extend back from the river, the principal one 

 of which is Wormley Creek (Plate XIX, figure 1). The marls are of 

 of Miocene (Yorktown) age. The Yorktown formation in Virginia has 

 a thickness of approximately 125 feet, and is described in detail on pages 

 158 to 166. A carefully measured section is given on page 161, to 

 which the reader is referred. The maximum working thickness of the 

 formation for cement manufacture at any one place in the vicinity 

 of Yorktown, will probably not exceed 30 feet above stream level. Its 

 general dip varies from 5° to 25° northwest, with an average of about 15°. 



Through the courtesy of Mr. H. E. Brown, Chief Engineer of the 

 American Cement Engineering Company, I give below analyses of the 

 marls occurring on the Jamestown Portland Cement Corporation's property. 

 The first four analyses of the marl given below are reported to represent 

 the entire face of the cliff shown in plate XIX, figure 1. Magnesia not 

 exceeding 0.5 per cent is reported as occurring in all of the samples. 

 These different layers are said to be found in practically all of the 

 exposures and in most of the borings which have been made on the property. 



Analyses of marls from the Jamestown Portland Cement Corporation's 

 property, Yorktown, Va. 



Constituents 



VI 



Silica (SiOa) I 24.06 



Alumina ( AIA) 3 . 29 



Ferric oxide (FcoOs) 4.47 



Calcium carbonate (CaCOs) .. 76.71 

 Magnesium carbonate ( MgCOa ) j 



Moisture I 



9.85 

 2.07 

 2.88 

 78.20 

 0.28 

 1.98 



I. A low grade marl ranging from 4 to 6 feet thick at the bottom of the deposit. 



II. Average sample of marl taken from a bed 6 to 10 feet thick immediately 

 above I. 



III. Average sample of marl taken from a bed 3 to 5 feet thick immediately 

 above II. 



IV. Upper bed of marl at top of deposit, overlain by sandy marls 2 to 4 feet 

 thick and too siliceous for use in Portland cement manufacture. 



V. Average sample of marl taken from a 10-foot exposure on Wormley Creek. 



VI. Average of samples collected from a 15-foot thickness of marl exposed about 

 500 feet from the bluff shown in plate XIX, figure 1. 



