76 REPORT OF THE SECRETARY. 



of casual study during the past summer by Mr. W J McGee, whose re- 

 ports may be summarized as follows : 



The uppermost deposit consists of a unipartite but heterogeneous layer 

 of gravel and loam, reposing unconformable* upon the several older 

 rocks of the region up to an altitude of 200 feet or more above tide. In 

 general, the layer thickens toward waterways and is attenuated toward 

 highlands, and the coarser materials predominate toward its base. 

 The gravels are in part quartzitic, small, and well rounded, and in part 

 of various sublocal rocks of all sizes up to fully four feet in diameter, 

 and little worn ; while the loam is sometimes of such fineness and local 

 homogeneity as to simulate the coarser varieties of loess. The whole 

 deposit is unfossiliferous, but is probably quaternary. 



Below these superficial gravels and above the crystalline schists and 

 gneisses of the easternmost Appalachian belt occur laminated sands 

 and plastic clays, unfossiliferous within the District so far as known, 

 but presumably of Cretaceous or Jurassic age. The series includes one 

 or more pebble-beds, consisting mainly of small and well-rounded quartz- 

 itic pebbles, which dip seaward with the arenaceous and argillaceous 

 strata, and soon disappear beneath the surface. Most of the eminences 

 of the region are outliers of these obdurate beds, isolated by erosion ; 

 and from each such eminence is a talus, tlerived from and more or less 

 closely resembling the undisturbed deposit, which descends the slopes' 

 and ranges, perhaps by imperceptible degrees, into the gravels of sup- 

 posed Quaternary age. This intermingling of similar deposits of widely 

 different ages is most puzzling, and greatly increases the difficulty of 

 the investigation. 



East of the District richly fossiliferous eocene rocks overlie the Cre- 

 taceous, but thus far they have been only partially examined. 



Beneath the superficial gravels sometimes occurs a ferriferous and 

 lignitiferous subaerial deposit, in some cases of age manifestly ap- 

 proaching that of the Cretaceous or Jurassic rocks upon which it re- 

 poses, and in other cases of so recent aspect as to be generally mistaken 

 for post-Quaternary accumulations. Local exposures of this discontin- 

 uous stratum may represent any portion, and the totality of exposure 

 apparently represents the whole of the period intervening between the 

 Jurassic and the Quaternary. Its iron ores are sometimes of economic 

 importance. 



Connected with the deposits is a complex system of terraces, both 

 fluviatile and littoral. Preparatory to the exhaustive study of these 

 terraces in their relations to the deposits with which they are associated, 

 and to the orographic movements which they attest, a topographic map 

 delineating the terraces in contours has been begun by Mr. S. H. Bod- 

 fish. 



Geologic map of the United States. — Pending the accumulation of ade- 

 quate material for the construction of the large-scale geologic map, a 

 small-scale map of the United States, embodying our present knowledge 



