Lewis.] 4:i4: ^jyi^y 15^ 



It is here iti contact with " EdgehillRock" (altered Potsdam). Ancient 

 eruptive norite rocks occur at Wa^me station, and the trap lies between 

 these older rocks (generally supposed to be Laurentian) and the hydro- 

 mica schists of the South Valley hill, which are entered by the railroad 

 west of here. 



The dyke enters Chester county in Easttown township, south of Devon 

 Inn, where it is in the so-called Laurentian area, and it is well shown at the 

 cross- roads at the corners of Easttown, Tredyflrin, and Radnor townships. 



Many other eruptive rocks, in the form both of dykes and of large bosses, 

 occur in the so-called Laurentian area, in the region south and west of 

 here, being largely developed in Chester county. These rocks are now 

 being studied microscopically by the author, who finds many of them to 

 be of high interest and capable of throwing much light on the difficult 

 geology of the region. The author has discovered an immense area of 

 eruptive garnet-bearing acid norite, with included dykes of diorite. dia- 

 base, gabbro, trap granulite, etc., and he has observed highly interesting 

 ettects of pressure, stretching and endomorphic changes. These will be 

 described in a subsequent paper. They are mentioned here merely to 

 state that these eruptive rocks have no possible connection with the much 

 later dyke which forms the subject of this paper. 



In Chester county the presence of trap along the southern border of the 

 South Valley hill in connection with serpentine, was briefly noted by 

 Rogers,* and Dr. P. Frazer in his report on Chester countyf has mentioned 

 a few isolated occurrences here and there, without, however, noting them 

 on the map. The only portion of the dyke placed upon his map is in 

 Easttown township, where, howevei', its line is not correctly represented. 



An interesting feature of the great dyke in Chester county, are a num- 

 ber of small "jogs," which continually displace the dyke short distances 

 to the northward. They are probably due to faults, since the same pecu- 

 liar structure is noticeable in the dykes of serpentine in the same region. 



Two such jogs are indicated in Easttown township ; one of them, half 

 a mile west of Devon Inn, the other somewhat more than a mile south-east 

 of Paoli. The dyke ends south-west of Devon Inn, but appears again 

 farther north on the road leading south from Reeseville, just south of an 

 exposure of serpentine. It now runs south-westerly to the Sugartown 

 road, at a point where the road forks southward to Leopard P. O. In this 

 vicinity large boulders, often four to five feet in length, are scattered 

 about on the soil, and are often resonant when struck. Again the trap 

 makes a jog of about half a mile to the north, as though again faulted. 

 Appearing in the north-west corner of Easttown township, on the road 

 leading from Leopard P. O. to Howellville, south of the cross-road to 

 Sugartown, it here divides the hydromica schists on the north side from 

 the hard syenite and hornbleudic rocks on the south side. The dyke keeps 

 in this geological position between the two formations all the way from 



* Geol. of Peiina., i, 11. 109. 



t C4j Second Geol. Survey of Peuua., pp. 



