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MARYLAND ACADEMY OF SCIENCES. 71 



appear buried in the sandrock, and may often be found resting 

 five or more feet below the surfixce of the ground. 



At the other extremity of the formation, next the ridge of 

 Baltimorean clays and sands which bar the approach of the 

 lower Tertiary beds to the basin of the Potomac river, we 

 observe the bluish or greenish-black marls, packed with the 

 remains of Eocene shells. These beds enter from the south- 

 east, pass across the head ravines of the western branches of the 

 Patuxent river, and enter above the base of certain hills Nvhich 

 are cut by tributaries of the head of Anacostia river. These 

 hills may be recognized by their chocolate-brown color and 

 softer, less coarse texture, than the similar ones which rise 

 towards the Bowen and Wheeler roads, within and beyond the 

 District of Columbia. 



On the property of Mr. Eobert W. Brooke the Eocene 

 greenish-black marl, packed with broken shells, is exposed at 

 the head of a tributary of the first large branch which crosses 

 Central avenue about three-quarters of a mile east of the 

 boundary of the District of Columbia. This outcrop of the 

 shell-marl is fully two hundred feet long and from seven to 

 twelve feet thick in the steep ravine of the brook. It ascends 

 into the hill in the direction of the west, and the brown soil 

 which overlies it rises nearly to the summit of the hill in which 

 it occurs, and then reappears in the hills pursuing their course 

 towards the residence of Mr. Brooke. The hill on which this 

 house is situated is composed also in great part of the same 

 chocolate-brown loam, and at a depth of twenty feet below the 

 surface there, only a few feet from the house, the same shell-marl 

 was met with in excavating for the well and found to have a 

 thickness of five feet. This is an exceedingly fat and almost 

 indurated shell-marl, which being so full of broken shells is in 

 fine condition to be employed as an agricultural fertilizer. As 

 the ravines of the next brook towards the west display no con- 

 tinuation of the chocolate-brown loam, and as they cut down 

 into the blue or reddish clays of the underlying Baltimorean 

 formation, we fail to find other exposures of the Eocene shell- 

 marl in that direction. Proceeding in the opposite direction, 

 however, about one mile towards the southeast, we again 

 encounter exposures of the same shell-marl on the head branches 



