1890.] NATURAL SCIEN'CES OF PHILADELPHIA. 107 



Ridley creek. Within this area are several outcrops of serpentine, 

 and scattered over a large part of its surface, amidst the soil, is the 

 honeycomb quartz which usually results from the alteration of 

 serpentine. This has given rise to the impression that the under- 

 lying concealed rock is serpentine. On Mr. Hall's map certain 

 areas are marked serpentine without question, and between the 

 areas so marked the map is lined green. ]\ly observations are 

 that in much of the area colored serpentine, while that rock may 

 exist, it constitutes but a small proportion of the area, while in much 

 of the lined portion serpentine does not exist. It is true that it is 

 impossible to define the areas of serpentine and other rocks in the 

 table-laud. The covering of soil is deep and the exposures very few, 

 but there are some that throw much light on the structure. The 

 region lies directly in the strike of the schists so well exposed near 

 Media and north of it and resembling the ^lanayunk series. INIr. 

 Hall, as already stated, believes these schists to lie at low angles and 

 to overlie the harder gneisses which appear over a large area further 

 west, but in the cut at INIedia, which Mr. Hall quotes in support of 

 this, ^ the alternations of differing materials certainly dip about 70° 

 to 90° southeast and I think this steep dip is, the rule, with very few 

 exceptions. 



The only section through this table land, and that by no means 

 satisfactory, is afforded by the excavations of the railroad, which 

 crossing the valley of Ridley creek by a very high bridge, passes 

 southwestward to Chester creek at a grade usually below the level 

 of the adjacent land. Decomposition of the rocks has, however, gone 

 on to so great a depth, that undecomposed rock is rare, and the walls 

 of the cuts, in many places are little more than loam, but there has 

 evidently been no transporting of material, the rock having decom- 

 posed in place. Usually the falling down of the upper part of the slopes 

 obscures the lower, but the excessive and violent rains of the past 

 season have washed the sides so that the rocks are very clearly 

 exposed, the stratification is clear and distinct, and the sections are 

 as convincing as if of hard rock. Indeed the opportunity for observa- 

 tion is exceptionally good. 



On the west bank of Ridley creek the railroad enters a cut about a 

 hundred yards from the bridge. This is in schists much like those on 

 the east with a strike about X. 40° E. and a nearly vertical dip. The 

 strike and the railroad tangent are here nearly parallel, the railroad 



1 Vol. C 5 p. 59. 



