1885.1 ^^ [Lewis. 



south, crossing the Little Neshamiuy creek uear the line bL-twcen "War- 

 minster and Warrington townships, and in Warminster township grad- 

 ually bending south-westward till it crosses the county line into Mont- 

 gomery county a mile and a half west of Warminster P. O. 



In Horsham township, Montgomery county, it passes through the upper 

 part of the village of Horshamville, crossing the Doylestown pike at the 

 tollgate. It crosses the Welsh road 100 ft. above the road leading from 

 Horshamville to Jarrettown, here forming a sharp hill. It is traced con- 

 tinuously from here to Jarrettown, where it runs under the school-house 

 and the Methodist Church. The ground is covered with boulders of trap, 

 giving the impression that the dyke is wide at this point. It here forms 

 a sharp hill, descending steeply to the north. A white Triassic sandstone 

 outcrops south of the dyke, between .Jarrettown and Dreshertown. 



A road running south-west from Jarrettown keeps along the dyke, which 

 for some miles forms a steep slope on its north side, and is readily recog- 

 nized. It crosses Susquehanna avenue about 100 ft. north of the road 

 from Jarrettown, and is conspicuous at the crossing of a small creek half 

 a mile further south-west. 



It is now approaching the edge of the Triassic formation, and in the 

 course of half a mile enters the Lower Silurian limestone valley of White 

 Marsh. " Mundock Hill," is a prominent feature ou the Township Line 

 road at the fork of the roads at the corner of Springfield, White Marsh and 

 Upper Dublin townships. The fragments of trap are here sometimes five 

 feet in length, and cover the side of the hill, extending down the Town- 

 ship Line road as far as Sandy run. 



Thus far the dyke has been followed without break in its course. .Just 

 here, however, at the corner of the three townships, there appears to be a 

 break of a few hundred feet, since no boulders of trap were noticed on the 

 road immediately west of the North Pennsylvania Railroad near Sandy 

 Run station. It may be that, the ground in the limestone valley now 

 entered being highly cultivated, the boulders have been removed by man, 

 and that the break is more apparent than real. For on the next road west- 

 ward, Church road, the dyke is clearly shown by numerous boulders at 

 the junction of the road to Flourtown. 



The dyke has now entered the Lower Silurian (Calciferous, Chazy and 

 Trenton) limestones of White Marsh valley, and cuts through them suc- 

 cessively without any apparent alteration either in the dip or in the char- 

 acter of the limestones. No evidence that the dyke was a line of fault 

 could be detected. The trap dyke continues across Springfield township 

 through Flourtown, as observed by Prof. Rogers and Mr. Hall, to the 

 Wissahickon creek, which it crosses at a point one-half mile north of the 

 Philadelphia county line. It here outcrops as a distinct wall sharply 

 rising out of the soil. It crosses a field in the stock farm recently owned 

 by A. Welch, and is well marked on the creek. It here offers an in- 

 structive example of a typical dyke. From here to Marble Hall, where 

 there is probably a small "jog" in its course, it is well marked as a line of 



PROC. AMEB. PHILOS. SOC. XXII. 120. JD. PRINTED SEPTEMBER 3, 1885. 



