PLANT LOCALITIES. 25 



tions. It possesses in Maryland the features which usually distinguish it 

 in Virginia, except that, so far as seen in Maryland, it is never cemented 

 together and indurated sufficiently to be called a sandstone. It is an in- 

 coherent sand of varying character, which has a current-bedded structure, 

 clay locally replacing the sand in various ways. 



In the city of Baltimore, on Covington and Bolt streets and in other 

 places, several feet of laminated gray shale and clay occur, interbedded 

 with argillaceous sand. This material shows on Covington street, at the 

 base of Federal Hill, a thickness for the whole mass varying from six to 

 twelve feet. This small group of strata, which on the whole is decidedly 

 argillaceous, rests on the typical sand of the lower Potomac, and forms a 

 member of this lower formation, which, although probably once generally 

 present, is now in most places removed by erosion. This argillaceous por- 

 tion of the lower Potomac in several places in the city, and notably on 

 Covington and Belt streets, has afforded some interesting plants. 



On this plant-bearing material, or, in its absence, on the typical sandy 

 material of the lower Potomac, we find resting everywhere in Maryland the 

 thick mass of clay and sandy clay above mentioned, which has been called 

 by Mr. Tyson and Professor Rogers "the Variegated Clays." Owing to the 

 common occurrence of iron ore in this group of strata, it is very commonly 

 called the "Iron-ore Clay." As stated above, this formation, at least for 

 the present, must be regarded as forming the up])er member of the Poto- 

 mac formation. It will in this memoir be called the "Variegated Clay 

 Group." 



This group has not been identified in Virginia, but it may possibly be 

 feebly represented at i\Iount Vernon and near Alexandria. The southern 

 limit so fiir as made out is Fort Washington. It shows itself in great force 

 in the vicinit}- of Baltimore, and has been traced by Mr. W J McGee as far 

 as the head of Chesapeake Bay, where extensive exposures are shown. 



How far it extends into Delaware and New Jersey and what relations 

 it bears to the plastic clays of the latter State remain to be determined. 

 It is separated from the lower Potomac below by an erosion horizon, and 

 it is composed certainly in part of the ruins of this group. From the 

 Eocene also above it is separated l)y an erosion horizon. So far as the 



