THE CALVERT FORMATION. 127 



found. The clay also carries at times considerable glanconite, so much that 

 locally it closely resembles the Eocene greensand. Such an occurrence 

 can bo seen at the base of the Wilmont bluffs below Port Conway on the 

 Rappahannock Eiver. 



Deposits of diatomaceous earth constitute the most striking feature of 

 the Calvert. These consist of countless millions of microscopic siliceous 

 tests of diatoms. They are so small that Ehrenberg estimated that one cubic 

 inch of Bohemian diatomaceous earth contained about 40,000,000 tests. 

 When the diatomaceous earth is pure it is white in color and very light in 

 weight. It, however, contains very frequently considerable argillaceous 

 material and then is apt to be blue in a fresh exposure but whitens by 

 contact with the atmosphere. It is often slightly discolored by the presence 

 of small quantities of hydrous oxide of iron. 



The diatomaceous earth beds of the Calvert extend across the states of 

 New Jersey, Delaware, Maryland, and Virginia. They have been pene- 

 trated in some of the deep wells at Atlantic City where they have a thick- 

 ness of about 300 feet. They are well exposed in Maryland at Herring Bay 

 and Pope's Creek. At Wilmont bluffs on the Rappahannock River they have 

 a thickness of about 50 feet in bold, almost vertical cliffs which present an 

 appearance almost as white as chalk when viewed from a distance. Other 

 good exposures are at Carter's Wharf on the Rappahannock River and along 

 Shockoe Creek in the eastern part of Richmond, where a thickness of about 

 20 feet is shown. The diatomaceous earth also passes frequently under the 

 names of infusorial earth, tripoli, Richmond earth, and Bermuda earth. 



StriJi-e, dip and thichfiess. — The strike of the Calvert formation is par- 

 allel to the line of outcrop, namely, north and south, while the beds dip to 

 the east at the rate of about 10 feet to the mile. The thickness of the beds 

 is about 200 feet. 



Areal distribution. — The formation makes its appearance in southern 

 New Jersey where it occurs at Shiloh and Jerico, crosses Delaware and 

 Maryland as a gradually widening belt, 10 to 30 miles in width, and con- 

 tinues as far south as Prince George County, Virginia, with a width fre- 

 quently greater than in Maryland. It is excellently exposed in hundreds of 

 places along the Potomac, Rappahannock, Mattaponi, Pamunkey, James, and 

 Nottoway rivers and their tributaries in the counties of King George, West- 

 moreland, Essex, Caroline, Hanover, Henrico, King and Queen, King Wil- 

 liam, New Kent, Charles City, and Prince George. It also occupies small 

 areas in a number of other counties both to the east and to the west of the 

 ones named. In the northern part of this area, particularly in Stafford 



