106 THE CRYSTALLED ROCKS OF CECIL COUNTY 



The contact plane of the gneiss and limestone is a plane of weak- 

 ness and is, therefore, locally at least, sometimes a thrust plane. 

 This is the case at the Chester County Poorhouse, seven miles west 

 of "West Chester, where the mica-gneiss has been shoved over the 

 limestone, cutting off some portion of the latter. At the Avondale 

 quarries (Baker's Station, Pa.) the reverse adjustment has taken place 

 and a thrust of the limestone against the mica-gneiss occurs. While 

 in an abandoned quarry only a quarter of a mile to the west of this 

 locality a conformable contact of gneiss and limestone can be seen. 

 As the mica-gneiss is approached the limestone shows inter stratified 

 mica-gneiss beds. 



That mica-gneiss conformably overlies limestone is also found to 

 be the case in the Doe Pun district and in several limestone quarries 

 west of West Chester. In short, in the numerous exposures of lime- 

 stone and mica-gneiss southeast of Chester Valley, conformable strati- 

 fication is the rule and adjustment is local and exceptional. 



RTo fossils have been found in these limestones. They have been 

 referred by the geologists of both the First and the Second Geological 

 Surveys of Pennsylvania to the period represented by the Chester 

 Valley limestone, i. e., to the Cambrian-Silurian period. They per- 

 fectly resemble these limestones, lithologically, chemically, in char- 

 acter of crystallization and in prevailing structure. Their exposures 

 are not far to the southeast of Chester Valley from which they are 

 separated by overlying mica-gneiss and Hudson schists. 



ISTot only does the Wissahickon mica-gneiss overlie limestone, which 

 furnishes no evidence of an earlier age than that of adjacent Cam- 

 brian-Silurian limestones, but where the mica-gneiss and Hudson 

 schists, which overlie the Chester Valley limestone, adjoin, there is a 

 perfect lithologic gradation between the two formations and an 

 apparent continuity of structure. This can be observed in the sec- 

 tions furnished by the east and west branches of the Brandywine. 



While in typical development they are very distinct in character, 

 when adjacent it is practically impossible to place a satisfactory line 

 of separation between the two formations. That this is the case in 

 the Susquehanna section in Cecil county has already been stated. 



