1880.] NATURAL SCIENCES OF PHILADELPHIA. 2tt 



Delaware Co., Pa., and from the Junction Railroad, above Girard 

 Avenue, Fairmount Park, associated with rhombs apparently of 

 lepidomelane or biotite, and also with quartz. 



On the Bryn Maivr Gravel Mr. Henry Carvill Lewis re- 

 marked, that since the presentation of his paper on the " Surface 

 Geology of Philadelphia and vicinity," he had been able to extend 

 the investigation then begun, considerably beyond the limits of 

 Philadelphia. The " Upland Terrace " has now been traced con- 

 tinuously from near Trenton, through Bucks, Philadelphia, and 

 Delaware counties, to beyond Wilmington in Delaware. As far 

 as could be judged, the clay comes up to a uniform level along 

 this terrace. It has been gratifying to find that the main charac- 

 teristics of the different deposits, recorded in the paper referred to, 

 are constant throuohout the whole of this region. 



The principal difficulty in the work has been want of topographi- 

 cal data. While within the limits of the city, the topographical 

 map of the Water Department had been of great service, but 

 beyond these limits elevations had to be estimated from occasional 

 railroad levels. Topography is an aid in all geological investiga- 

 tions, but in the study of surface geologj^ it is a necessity. 



It is now desired to call attention to the great development of 

 the Bryn Mawr gravel in Delaware, and to the indications of its 

 assuming an important position in the geology of the Southern 

 States. In Bucks County, north of Philadelphia, the formation 

 has been recognized but scantily, but as we go south of the city 

 it increases largely in extent. Numerous hills in Delaware Co. 

 have been found to be capped by this formation, and in northern 

 Delaware it covers the gneissic hills in patches several miles long 

 and comes close to the river. 



The Upland Terrace, after crossing the Delaware State line 

 about two and a-half miles back from the river, gradually ap- 

 proaches it, until near Bellevue Station, P. W. and B. R. R., its 

 base is but half a mile from the river. It forms the upper portion 

 of Wilmington, and then trends S. E. towards Baltimore, north of 

 the railroad and away from the river. In the neighborhood of 

 AVilmington the Bryn Mawr gravel lies directly upon and back of 

 the Upland Terrace, which is here about 300 feet high. It is 

 abundant to the southeast of Tallyville, Del., covering a large 

 tract of country, and it appears on the hills on both sides of the 

 Brandy wine in the neighborhood of Dupont's Powder Mills. It 

 is found on the Philadelphia and Wilmington Turnpike, two miles 

 northeast of Wilmington, and one mile from the river. In many 

 places it is five feet deep, and it seems less eroded than in Penn- 

 sylvania. It consists of sharp pieces of Mt. Holly conglomerate 

 and iron sandstone with well-rounded pebbles of quartzite and of 

 Potsdam sandstone, being identical with that of Chestnut Hill 

 and Br^n Mawr. 



