Maryland Geological Survey 65 



often septarian, and their cavities are commonly lined by brown velvety 

 masses of siderite crystals, which change to brown hematite on exposure 

 to the air. The materials of the Arundel formation are so strikingly 

 homogeneous, as compared with those of the underlying Patuxent and 

 overlying Patapsco, that its deposits have served as a datum plane 

 for Potomac stratigraphy in Maryland, and the iron produced is prized 

 for its high tensile strength. 



The clays are commonly free from grit, but are at times sandy, and to 

 the landward the sand may predominate, as in the vicinity of Washing- 

 ton. The clays are not infrequently pyritous and gypseous, both minerals 

 commonly occurring in druses. The gypsum druses frequently line the 

 septarian nodules but also occur free in the clays, as at Spring Gardens. 

 The clays are in general unctuous or " fat," and are an important resource 

 for brick, terra cotta, and pottery manufacture. They have been worked 

 for these purposes at several points, but will undoubtedly be much more 

 extensively employed in the future. 



The siderite deposits known locally as " oolite ore " are changed com- 

 monly at the surface, and in the clays poor in carbon to greater depths 

 into hydrous oxide of iron or limonite, known locally as " brown ore." 

 These ores have been jnined since early Colonial days, one furnace at 

 Muirkirk being still in operation. 



The lignitic element in the deposits, which gives to the clays their 

 characteristic drab color, at times becomes so pronounced as to form 

 well-deiined lignite beds, which have been locally used as fuel, as at 

 Soper Hall Hill, Anne Arundel County. The lignite is at times finely 

 scattered through the clays, at other times trunks, limbs, twigs, and 

 leaves are found well preserved, the stumps in some instances being 

 found in erect position with their roots intact as they grew. At times 

 the woody fibre may be partly replaced by siderite or by pyrite, as at 

 .Reynolds' iron mine at Hanover. 



Strike, Dip and Thickness. — The strike of the Arundel formation 

 is essentially parallel to that of the Patuxent toward the north, being 

 about northeast to southwest and gradually becoming more nearly north 

 and south as southern Maryland is reached. 



