REVIEWS 549 



lower clayey member which is shown south of South River. The clay is a fine grained 

 material with occasional streaks of sand, and is in places twenty feet thick. It is 

 fairly plastic, abundant, and exposed near tidewater; suitable for use in the manufac- 

 ture of pressed brick and possibly of paving brick. 



The Jurassic and Cretaceous of Maryland are divisible as follows : 



Rancocas 

 Upper Cretaceous -J Monmouth 



Matawan 



r, . \ Raritan 2 ^ 



Lower Cretaceous < „ , 



/ Patapsco n . 



. ,,. Arundel f Potomac group. 



Jurasslc ( ? ) • J Patuxent. j 



The Upper Cretaceous formations include no clay deposits of economic value ; 

 but the Potomac group is of great importance. The Raritan ' consists usually of 

 white sands and light colored clays, and reaches a thickness in central Maryland of 

 one hundred 2 feet. Raritan clays are found in Cecil, Kent, Harford, Baltimore, 

 Anne Arundel, Prince George's, Howard, and Montgomery counties. They are 

 worked at several points for use in pressed brick manufacture ; while potter's clay 

 occurs in places. The Patapsco includes brightly colored mottled clays, with light 

 sands and clays, and has a maximum thickness of about 200 feet. In Cecil county a bed 

 of bluish stoneware clay often underlies the variegated clays. Refractory clays, as well 

 as brick clays, occur, and the Patapsco clays have been exploited to a considerable 

 extent. The Arundel contains lenses of bluish, siliceous, plastic clays, often carrying 

 iron concretions. The clays are largely used in the manufacture of common and 

 pressed brick, terra-cotta, roofing tile, and common pottery. The Patuxent consists 

 largely of sands, with occasional beds of sandy clay, and is, in this regard, the least 

 important member of the Potomac. 



Shales from the different formations of the Carboniferous, Devonian and Silurian 

 have been tested and found suitable for various uses. At present only two of these 

 formations furnish material of economic importance, these being the Pottsville forma- 

 tion of the Carboniferous and the Jennings formation of the Devonian. Both flint 

 clays and plastic clays occur in the Pottsville formation, the well-known Mt. Savage 

 fire clay being an example of the former. The beds have been opened near Frost- 

 burg, at Mt. Savage and west of Ellerslie ; and will probably become of even greater 

 economic importance. Shales of the Jennings formation are extensively used in the 

 manufacture of paving brick at Cumberland. 



Kaolin occurs at many points in the area underlain by Algonkian rocks, notably 

 in Cecil county. These residual clays, having been derived from feldspathic rocks low 

 in iron-bearing minerals, are light colored and burn white. Only one company is 

 actually at work mining and washing this material, but the industry will probably 

 increase rapidly in importance. Owing to the presence of Patuxent and Columbia 

 beds, the kaolin is rarely exposed at the surface, the necessary stripping varying from 

 two to forty feet. 



1 On page 397 the Raritan is excluded from the Potomac, but is included in it on 

 pages 399 et seq. — E. C. E. 



2 Elsewhere in the volume stated as 500 feet. — E. C. E. 



