1912.] NATURAL SCIENCES OF PHILADELPHIA. 109 



APPARENT SUN-CRA.CK STRUCTURES AND RINGING-ROCK PHENOMENA IN 

 THE TRIASSIC DIABASE OF EASTERN PENNSYLVANIA. 



BY EDGAR T. WHERRY, PH.D. 



The rocks deposited during the Triassic period in eastern North 

 America, variously known as the New Red, Newark, and Jura-Trias, 

 •cross the State of Pennsylvania in a northeast-southwest strip 

 averaging twenty miles in width. While they have in the past been 

 the subject of considerable investigation, some of their most interest- 

 ing features are as yet undescribed. 1 



About the middle of the 20,000 feet of sediments representing the 

 period in Montgomery County there occurs a sill of diabase, which, 

 although greater in size than the Palisade sheet of New Jersey, has 

 attracted far less attention since it does not happen to be so located 

 as to give rise to striking scenic effects. The total length of outcrop 

 of this sill being over 40 miles, it would be strange if there were not a 

 few exposures of its contact relations with the sediments, even in the 

 absence of a great metropolis nearby as an inducement for railroad 

 companies to pierce it by numerous cuts and tunnels; and, in fact, 

 its upper contact has been observed at several localities northeast 

 of the city of Pottstown. The rock surfaces exposed at these places 

 by the removal of the metamorphosed shale beds present a rather 

 startling appearance, being crossed by a rudely hexagonal network 

 of light-colored lines, closely resembling sun-cracks such as are 

 frequently found in the sediments. The best exposure, a photograph 

 of which is shown in figure 1, Plate II, is on the east side of a road, 

 opposite the house of Alexander C. Minshall, one-half mile north of 

 Neiffer Post Office, which lies about three miles north of Limerick 

 Square and two miles west of Zieglersville. 



1 The writer has been engaged for several years in studying the portion of this 

 area lying east of the Susquehanna River, and has previously published two 

 papers upon it: "The Newark Copper Deposits of Southeastern Pennsylvania," 

 Econ. Geol., Ill, 726-38, 1908; and "Contributions to the Mineralogy of the 

 Newark Group in Pennsylvania," Thesis, University of Pennsylvania, 1909, 

 Trans. Wagner Free Inst. Science, VII, 1-23, 1910. An abstract of this paper 

 was read at the Pittsburgh meeting of the Geological Society of America, Decem- 

 ber, 1910, and published in Bull. Geol. Soc. Amer., XXII, 718, 1911; and in 

 completed form it was presented at the meeting of the Academy in association 

 with the Mineralogical and Geological Section, May 21, 1912. 

 1! 



