» Upon xohich the City of Philadelphia stands. 365 



level, and has not been disturbed by cultivation, is generally from 

 four to six inches deep. 



2d. When tlie situation is not very low, ten or twelve inches 

 of a yellow, tough, silico-argillaceous earth, called " loam." When 

 the situation is very low, this is of a gray colour. 



3d. In some places, gray and yellow sand and clay; the sand 

 sometimes coarse, and at others nearly impalpable ; silicious peb- 

 bles, from the size of an egg to mere grains, are found here. In 

 other places, under the yellow loam, the substance becomes hard 

 and sandy, first of a slightly brown colour, and afterwards 

 changing to gray sand. The depth of this stratum varies from 

 three to six feet. Between this and the next stratum is some- 

 times found six or seven inches of black clay, in the lower side 

 of which embedded pebbles are commonly found, some as large 

 as paving stones. 



4th. A fat, tenacious, and plastic clay, containing very little 

 silicious sand, and very few pebbles. The depth of this varies ; 

 it is sometimes six or seven feet. Not more than one fourth of 

 the site of Philadelphia has any of this stratum ; where there is 

 no clay, the place is supplied by sandy loam, mostly running into 

 gray colour, as it approaches the next stratum. When the clay 

 is regular, its depth is, on an average, three to four feet. This 

 clay is what is called potter's clay. The three last strata mixed 

 together form what is called brick clay. When the potter's clay is 

 superabundant, it is sold to the potters, or reserved to mix with 

 the sand and loam of other places, to make brick clay. 



5th. A yellowish, and frequently dark brown mixture, partly 

 chemical, and partly mechanical, of silicious, argillaceous, and 

 ferruginous particles, passing into a sort of imperfect iron ore. 

 This seldom exceeds in thickness two or three inches. 



6th. Gravel, containing boulders as before stated. The gravel 

 is coloured red with iron, and contains a small quantity of clay, 

 which imparts to it an adhesive quality, and renders it a fine ma- 

 terial for making gravel walks, or covering roads. Near the 

 Schuylkill the boulders are large, and the gravel coarse ; but the 

 boulders gradually diminish in size and quantity, and the gravel 

 gradually becomes finer as you approach the Delaware river. At 

 Seventh street it is a fine sand. It is difficult to ascertain the 

 depth of the gravel ; water is generally found in it, at the depth 

 of sixteen feet. 



