1885.] 44^ [Lewis. 



An interesting exposure of the trap in place was noticed at a short dis- 

 tance north of the spring, at Spring Mill, where the dylce is cut through 

 hy the creek. This is the only locality in Pennsylvania where the author 

 has observed basaltic or columnar structure in this dyke. Large columns 

 of trap lie here, nearly horizontally, several of them showing six sides, 

 and one of them five feet in diameter. Their nearly horizontal position 

 is, of course, due to the fact, that the columns lie at right angles to the 

 cooling surfaces. As the cooling surfaces are here nearly perpendicular, 

 the columns have taken a horizontal position. The columns appear to 

 have an inclination of about eight degrees to the horizon, indicating, per- 

 haps, that the dyke varies that much from the perpendicular. Although 

 columnar structure has not been noticed elsewhere in this dyke, another 

 structure characteristic of eruptive rocks is seen everywhere along the 

 whole line. Concretionary structure, which, sometimes associated with 

 columnar structure, but oftener observed alone, is due to the same con- 

 traction of the cooling mass, which sometimes forms columns, is a very 

 common feature of the dyke. Though not apparent in the fresh rock, 

 this structure is readily seen during the process of weathering, producing, 

 finally, the rounded boulders that have already been described. 



At Conshohocken, the dyke is finely exposed in the town and forms a 

 conspicuous escarpment on the river bank, well seen from the railroad. 

 At the time that the new Schuylkill Valley Railroad was being built, much 

 was said about the difficulty experienced by the contractors in cutting 

 through this remarkably tough rock at Conshohocken. 



Crossing the Schuylkill, in the bed of which it is seen, as described by 

 Professor Rogers, and being well shown on the west side of the river 

 where cut through by the Philadelphia and Reading Railroad in West 

 Conshohocken, the dyke follows a long straight line through Lower Meriou 

 township, Montgomery county, as mapped in Report C®, Second Geologi- 

 cal Survey. It is well shown near jNIechanicsville, where cut through by 

 Gulf creek. In this portion of its course the dyke either cuts through or 

 runs along the southern boundary of the hydromica schists (Hudson 

 River Age?) of the South Valley hill. A few hundred feet west of Me- 

 chanicsville, it is in contact with highly metamorphic limestone, a continu- 

 ation of that which occurs in Cream valley. 



Immediately west of here, at the head waters of Gulf creek, it is in 

 contact with what appears to be lower Cambrian sandstone, here highly 

 altered into a slaty rock ringing like metal when struck. The strata 

 stand nearly perpendicular with a steep north dip, and are much altered. 

 The metamorphism is believed by the author to be of much more ancient 

 date than the time of the eruption of this trap. 



After crossing the stream, it enters Delaicare county, Radnor township, 

 and crosses the Pennsylvania Railroad at a point about a quarterof a mile 

 west of Wayne station (at mile-post 15-338). Numerous loose boulders 

 occurred here iu the soil, but have recently been removed for railroad 

 ballast. 



