108 THE CRYSTALLINE ROCKS OF CECIL COUNTY 



underlying limestone and quartzite, is separated from the Paleozoics 

 on the northwest by a fault and that it is pre-Cambrian. If this is 

 the case the apparent conformity of the Calciferous Peach Bottom 

 slates and the Cecil county mica-gneiss is not the true relation of the 

 two formations. A thrust fault must separate the mica-gneiss from 

 the slates. This must also be the case with the Hudson schists and 

 the mica-gneiss. While no evidence has yet been found of such a 

 fault, and on the contrary proof of stratigraphic conformity has accu- 

 mulated, there is one fact which may indicate a time break between 

 the eastern gneisses and the western Paleozoics: 



The apparent confinement of eruptive material to the eastern 

 belt of gneisses is suggestive of a time interval between the forma- 

 tions. Professor George Huntington Williams 1 referred the eastern 

 gneisses to an earlier age (pre-Cambrian) than the western Paleo- 

 zoics of the Piedmont Plateau of Maryland. He emphasizes the 

 greater degree of metamorphism exhibited by the former formations. 

 This difference in degree of metamorphism does not deserve 

 emphasis in Pennsylvania. The quartzite and crystalline limestone 

 interstratified with the Wissahickon gneiss are no more thoroughly 

 metamorphosed than is the recognized Paleozoic quartzite and crys- 

 talline limestone. I^"or does the micaceous phase of the gneiss differ 

 in the degree of metamorphism exhibited by it from the mica schist 

 of Hudson age. 



Positive proof for or against the Paleozoic age of the Wissahickon 

 mica-gneiss is lacking. Until evidence for or against a faulted struc- 

 ture has been procured, or fossils are found in the interstratified 

 limestone, the age of the Wissahickon gneiss and hence of the Cecil 

 county mica-gneiss must be considered as undetermined. 



It is either pre-Cambrian or Lower Silurian. 



THE ERUPTIVE ROCKS. 



The question next occurs, are the igneous formations intrusive in 

 the mica-gneiss or do they altogether underlie it? 



The meta-peridotites and pyroxenites show the usual character of 

 intrusives; dike-like exposures, apophyses, the occurrence of discon- 



iBull. Geol. Soc. Amer., vol. ii, 1891, p. 316. 



