204 CLARK AND BIBBINS — GEOLOGY OF THE POTOMAC GROUP 



Maulden Mountain, immediately above Giller's Hole. 



Feet 



Columbia 1. Loam and gravelly loam. . 6 



Matawan 2. Massive micaceous glauconitic sands, mottled with brown, 



oxidized, and more or less indurated near the top 30 



3. Loose, lighter-colored sands, with less glauconite, oxidized at 



the surface (brown flecks) 6 



4. Sharp white and yellow sands, indurated at the base 3 



5. Yellow, red, and ash-colored sandy clays 2 



6. Loose sands, micaceous and more argillaceous toward the 



base 15 



7. Lens of loose carbonaceous and pyritous sandy loam, grav- 



elly at base 6 



Raritan -8. Lens of stratified, iron-stained, at times pebbly " fuller' s- 

 earth " clay, occasionally lignitic (dicotyledenous? stems), 



indurated at base 3-10 



9. Light buff and brown crossbedded sands, coated with " ful- 

 ler's earth" 25 



10. Ledge of corrugated iron sandstone 2-9 



11. Sands similar to 9, brightly iron-tinted in the middle and lower 



portions and containing white clay pebbles and pellets ; un- 

 dulating base 15 



Patapsco 12. Massive variegated and drab lignitic plastic clays, the latter 

 at times containing iron carbonate ; obscured by talus and 

 land slip 20 



Total thickness 138 



Economic products. — The economic products of the Raritan formation 

 in Maryland include building and glass sands, quartzose and ferruginous 

 sandstones for building purposes and road metals, clays used in the man- 

 ufacture of buff face brick and pottery, drab clays used for stoneware 

 and modelling, and variegated and red clays also used at times for pot- 

 tery. White sandy clay and white arkosic sand, known as " fuller's 

 earth," occur on a large scale, and are in local use for polishing metallic 

 surfaces. This will doubtless prove to possess wider economic possi- 

 bilities. Massive pyritic deposits occur in workable quantities at a few 

 points, notably at cape Sable, on the Magothy river, where they were 

 mined early in the last century, and alum, copperas, and sulphuric acid 

 produced. The amber already mentioned as occurring at this station 

 is only of scientific and historical interest. 



Interpretation of the Potomac Deposits 



In their former paper the authors pointed out that Potomac deposition 

 was probably preceded by extensive baseleveling of the eastern side of the 

 continent, with widespread rock disintegration. Stimulated by elevation 



