PAT APS CO FORMATION 199 



rugated iron sandstone (plate 24, figure l),at times conglomeritic. The 

 " variegated clays," which commonly exhibit a great variety of excep" 

 tionally rich and delicate tints in extremely irregular "pied " patterns, 

 often grade downward or horizontally into massive or stratified chocolate, 

 drab, and black clays which are often lignitic, more or less pyritous, and 

 occasionally iron and leaf bearing. The sands sometimes contain decom- 

 posed feldspar grains, as well as pellets and balls of white clay. They 

 are frequently crossbedded, though less strongly marked than in the 

 Patuxent formation. Red ocher, known as " paint rock " or " paint 

 stone," occurs near the base and summit and sometimes within the for- 

 mation, while flakes of sand}^ and ocherous limonite with botryoidal 

 inferior surfaces are not uncommon at various horizons.' The variegated 

 clays often contain great numbers of small flattened pieces of limonite, 

 quite uniform in dimensions. When these are brought to the surface 

 by erosion, they form the resistant caps of innumerable miniature ero- 

 sion towers which beset the crests and slopes of the verdureless " bad- 

 land " areas, well shown at Bald hill, Prince George's county. 



Organic remains. — The flora of the Patapsco formation includes ferns, 

 cycads, conifers, monocotyledons, and dicotyledons, the last still consti- 

 tuting an inconspicuous element as compared with the other types repre- 

 sented. The range of genera and species is in the main limited, the 

 grade of organization still moderately low, and the number of individuals 

 scarcely greater than that of the preceding formation. At one station, 

 however, near the summit of the formation there occurs a profusion of 

 apparently a single species of leaf resembling Platanus. 



The known fauna of the Patapsco formation is limited to a single, 

 much worn, silicifled, dinosaurian limb-bone, which was found at the 

 surface and may have been redeposited from the Arundel. 



Strike, dip, and thickness. — The strike of the Patapsco formation corre- 

 sponds practically to that of the formations below it. The normal dip of 

 its basal beds is from 35 to 40 feet per mile. This rate, as in the case 

 of the preceding formations, is strongly emphasized to the landward at a 

 few points, notably in the Principio area, where the formation reaches to 

 the Fall line. 



The thickness of the Patapsco formation in central Maryland at the 

 point where its summit descends below tide is estimated at 240 feet. The 

 greatest exposed thickness occurs at Grays hill, Cecil county, where it 

 reaches 100 feet. At Broad Creek hill, south of New Glatz, Prince 

 George's county, 80 feet of Patapsco clays are exposed. 



The Arundel- Patapsco boundary. — The Arundel-Patapsco line repre- 

 sents very nearly the line of demarkation between the iron-bearing clays 

 proper, with their barren equivalents and that great mass of variegated 



