GEOLOGY OF DELAWARE COUNTY. 405 



does not present a sinRle stratum. Our rocks were either formed before such 

 beings were called into existence, or every trace of their remains has been 

 effaced by the great subterranean heat to which they have been suljjecled. 

 Resting u'nconformably on these rocks, we have in some parts of the County, 

 deposits of clay, gravel, or sand, formerly, though improperly, called dUuvtum, 

 while in other sections they are covered with the earthy results of their own 

 decomposition. 



Professor Henry D. Rogers, our State Geologist, designates our rocks by the 

 terms Hypozoic or Gneissic. Of these rocks lie found three districts within the 

 limits of his survey; \.\\c Jirsl or most southern of which embraces substantially 

 the whole of Delaware County, and " ranges from the Delaware River at Trenton, 

 to the Susquehanna, south of the State line." 



Mr. Rogers informs us, that "this most southern belt of our crystalline strata 

 makes its first appearance at a spot in New Jersey, about six miles N. E. of 

 Trenton, where it emerges from beneath the margin of the overlapping Mesoic 

 Red Sandstone." Its lower or southern margin, he says, "crosses the Delaware 

 River a short distance below the bridge at Trenton, and passes by Bristol, Phila- 

 delphia, Chester, and Wilmington, being separated from the river by a narrow 

 strip of diluvial and alluvial deposits, which only in a few places exceed one 

 mile in width. The northern boundary commencing at the same point in New 

 Jersey, crosses the Delaware about a mile and a half above Trenton, and ranges 

 in a somewhat undulating line to Sandy Creek, about a mile east of the 

 Wissahickon." 



" W. of the Wissahickon, the northern edge of this zone of gneiss, ranges just 

 S. of Barren Hill; crosses the Schuylkill a little below Spring .Mill, passes about 

 a mile and a half S of the Paoli, and terminates near Boardsley's Run of the 

 West Branch of the Brandywine, and not far from the Chester County Poor-House. 

 W. of the Brandywine the gneissic rocks sink under the altered primal strata, in 

 a succession of anticlinal fingers on slender promontories." 



It will thus be seen that the whole County is included in the first gneissic 

 district of -Mr. Rogers, except a very small part of Radnor township, which ex- 

 tends into the South Valley Hill, and which he includes in his next higher 

 division of rocks termed Azoic. As this division is alike destitute of organic 

 remains as the gneissic, and was established merely on the fact that it is less 

 crystalline than the former, the ditierence between the two belts is of no practical 

 importance, and the line of junction very frequently cannot be determined. 



In describing our rocks in detail, Mr. Rogers has divided his southern Gueissic 

 district into three subdivisions. I will not follow him in this, because his con- 

 clusions were mostly drawn from examinations made on the Schuylkill, which 

 frequently do not hold good when extended into our County, and because the 

 accompanying map will suffice to show the location of each variety of rock much 

 better than it could be given in words. It must be remembered, however, that 

 our strata are not continuous for any great distances; that they frc(iueutly alter- 

 nate, and that the constituents of the same stratum will be ditterent in its dif- 

 ferent parts. It will therefore be understood, that the color adopted to indicate 

 on the map the presence of any particular rock, is not intended to convey the 

 idea that that rock is exclusively present in the particular locality represented by 

 the color. It merely shows a /jrcrfomi/if/Hce of the rock indicated by the color. 

 This is the best that can be done, where the strata are so extremely variable. 



The direction of the strata and their dip are also exceedingly variable. The 

 general or average direction may be given as nearly north and south, and the 

 dip a little towards the west. But frequently the strata are nearly vertical or an 

 opposite dip is visible. 



Commencing on what is known as the '• Line road," at its junction with the 

 old Haverford road, in the 24th ward of the City of Philadelphia, the presence 

 of a trap dyke can be traced continuously for some distance into the township 

 of Springfield. The gneiss rock on either side of this trap has undergone a 

 striking metamorphism. It appears to have been originahy constituted of the 

 nsual ingredients — quartz, mica, and feldspar, or sometimes with the mica re- 

 placed by hornblende. The eflect of the protruded trap has been, to aggregate 



