196 CLARK AND BIBBINS — GEOLOGY OP THE POTOMAC GROUP 



The normal dip of the formation is 40 to 50 feet per mile. At Balti- 

 more it appears to be somewhat less than this. There is a well marked 

 increase in dip to the landward, along or near the Fall line, as in the case 

 of the Patuxent the average rate of descent at twenty stations being about 

 72 feet per mile, the range being from 34 to 190 feet. . Abingdon, 34 feet ; 

 Joppa, 40 feet ; Loreley, 93 feet; Stemmers run, 53 feet; Homestead 

 (Baltimore), 60 feet ; Loudon park, 50 feet ; Relay, 80 feet, with one local 

 dip of 133 feet and another of 190 feet; Hanover, 106 feet; Jessups, 40 

 feet ; Annapolis Junction, 80 feet ; Savage, 50 feet ; Muirkirk, 62 feet ; 

 Beltsville, 60 feet ; Branch ville (local), 80 feet ; Riggs' mill, 40 feet ; Brook- 

 land, 40 feet; Washington, 64 feet. 



The thickness of the Arundel formation ranges from to 125 feet or 

 more. The greatest thickness of the exposed beds occurs in the iron mines 

 above Hanover and in the Muirkirk area. The data regarding the thick- 

 ness of this formation to the eastward, where its summit descends below 

 tide, are from artesian well borings, which must be accepted with caution. 

 They tend to show that the formation does not thicken perceptibly along 

 the dip beyond a certain point, and that the deposits lack horizontal con- 

 tinuity. 



The Patuxent- Arundel boundary. — The Patuxent- Arundel boundary is 

 on the whole the most clearky defined within the Potomac group. The 

 occasional occurrence near the landward margin of the Patuxent forma- 

 tion of beds of drab lignitic clays, which are at times slightly iron-bear- 

 ing and of massive red and variegated clays, although all of relatively 

 small extent, has caused at a few points some uncertainty as to the true 

 boundary line, owing to the fact that the strong increase in the dip of 

 the Patuxent strata has brought its beds of clays into range with the nor- 

 mal dip of the Arundel clays. The difficulty has been further compli- 

 cated by the occurrence of landslips, apparently dating back to the 

 Quaternary, by which workable masses of Arundel iron ore clay were 

 precipitated from elevated positions above the Patuxent sand and gravel, 

 and are now more or less obscured by more recent sediments. 



When organic remains can be found the problem is much simplified, 

 since the floral and faunal contrasts are well marked. In the Patuxent 

 terrane both plant and animal remains are comparatively scant, its di- 

 cotyledons being limited to a few very primitive types. In the Arundel, 

 on the contrary, the remains of both plants and animals are relatively 

 abundant. Normal, although still simply organized, dicotyledons are 

 not uncommon. Dinosaurian remains, wanting in the Patuxent terrane, 

 are comparatively common in the Arundel. 



Though now for the first time cartographically shown, suggestions of 

 the occurrence of the Arundel- Patuxent boundary have appeared from 



