194 CLARK AND BIBBINS — GEOLOGY OF THE POTOMAC GROUP 



found in the differentiation of certain massive red and variegated clays 

 near and at the base of the Patuxent formation from residuals of similar 

 character from which they were evidently derived by redeposition. 

 Other residuals closely resemble Patuxent arkosic sands. In cases of 

 imperfect exposures these resemblances are at times the occasion of con- 

 siderable uncertainty. 



Characteristic local section. — Tyson published in 1860 a record of the 

 strata penetrated by an artesian well at " Smith's distillery," situated 

 on Northwest harbor, Baltimore, which is an entirely typical Patuxent 

 section. After passing through 52 feet of river mud, the well penetrated, 

 at 42 feet below tide : 



Feet 



1. Sand, gravel, and bonlders. 6 



2. Hard blue clay , 9 



3. Red clay 6 



4. Red ocher 5 



5. White sand 4 



6. White clay 32 



7. White sand and gravel, water-bearing 8 



8. White clay 3 



9. White sand, gravel, and boulders, water-bearing 7 



10. Gneiss rock 



Total thickness 80 



The "boulders " of the first and ninth members are doubtless cobble. 



Economic products. — The Patuxent formation, which is on the whole 

 an arenaceous terrane, yields .building and other sands (plate 24, figure 

 2) on* a large scale, and road metal in the form of arkosic gravel and iron 

 sandstone and conglomerate. It also yields white and ferruginous clays 

 to some extent as well as black lignitic clays which are in use as a base 

 for pigments. Red and yellow ochers also occur. The drab clays con- 

 tain a very few workable beds of iron carbonate. 



ARUNDEL FORMATION 



Name and lithologic characters. — The Arundel formation receives its 

 name from Anne Arundel county, Maryland, where the deposits of this 

 horizon are typically developed and well exposed. 



The deposits consist chiefly of large and small lenses of drab or iron- 

 tinted clays. These clays are frequently iron-bearing, the varieties 

 being an earthy spathic ore, occurring in concretions, flakes, geodes, and 

 layers, often in many courses. They are also at times pyritous and oc- 

 casionally gypseous. The clays may be either massive, exhibiting 

 slickensides surfaces, or laminated. In the latter case they are usually 



