MARYLAND GEOLOGICAL SURVEY 



107 



The following section illustrates the structural relations of the 

 Wissahickon mica-gneiss farther north: 



Fig. 7. — Generalized section from Merion, Pa., to Haverford, Pa., showing possible 

 structure of crystalline formations. 



NF. = Newark Formation. S. = Serpentine. 



H S. = Hudson schists. W M-G. = Wissahickon Mica-gneiss. 



Gb-Gn. = Gabbro-gneiss. C-S L. = Cambrian-Silurian limestone. 



I = Diabase. 



The key to the structural relations of these formations is to be 

 found in Pennsylvania and more detailed investigation will either 

 establish the conclusions reached in a. rapid preliminary survey 

 without the aid of topographic maps, or furnish other stratigraphie 

 details. At present the facts seem to warrant the following conclu- 

 sions: 



The mica-gneiss of Cecil county is a geologic unit, though form- 

 erly regarded as two distinct formations: a " hydromica schist" in 

 the west and a " gneiss " in the east. It is the same formation as 

 the Wissahickon gneiss of Pennsylvania. The Wissahickon gneiss 

 conformably overlies limestone, which is presumably, though not 

 proven to be, Cambrian-Silurian. It adjoins, in southeastern Penn- 

 sylvania, mica-schist, which has been determined by fossils to be of 

 Hudson age. With this Hudson schist it shows continuity of struc- 

 ture and a lithologic graduation. The Wissahickon mica-gneiss shows 

 no greater metamorphism than the adjacent Paleozoics and no closer 

 folding. Unsymmetrical open folding is the rule, with a steep cleav- 

 age dip to the southeast in both Paleozoics and mica-gneiss. 



If these conclusions stand, the Wissahickon mica-gneiss and its 

 continuation in Cecil county must be referred to the Hudson divi- 

 sion of the Lower Silurian period. This is in accord with the occur- 

 rence, as a conformable lower member of the series, of the Peach 

 Bottom slates, which have been referred to the Calciferous. 



The alternative conclusion is that the Wissahickon gneiss with its 



