104 THE CRYSTALLINE BOCKS OF CECIL COUNTY 



Unsymmetrical overturned folds, as shown in Figure 5, are dis- 

 played, on a small scale, at Haines and at other localities. 



These minor folds may illustrate in miniature the large scale 

 folding of the entire formation. 



There is no recurrence of lithologic units, by means of which the 

 thickness of the formation can be ascertained, but the dips indicate 

 more or less unsymmetrical overturned folds. The vertical beds 

 may frequently represent the northwest limb of the folds, as is sug- 

 gested on the section accompanying the geological map. 



Ace. — The mica-gneiss of Cecil county contains within itself no 

 clue to its age. The solution of this problem must be found in the 

 stratigraphic relations which the formation may sustain to fossil- 

 bearing sediments. 



Such stratigraphic relations do not exist in Cecil county, but are 

 displayed in Pennsylvania and Delaware. The relation of the Cecil 

 county mica-gneiss to a similar formation in these states must there- 

 fore be indicated. 



The gneisses of the Philadelphia belt of crystallines (Chestnut 

 Hill, Manayunk and Philadelphia schist and gneisses) are divisible 

 into a mica-gneiss of sedimentary origin, and several, presumably 

 intrusive, igneous bodies; peridotites and pyroxenites, largely repre- 

 sented now by serpentines, gabbro and norite and two granite-gneisses. 

 The formation into which these igneous rocks have intruded is a mica- 

 gneiss, 1 stratigraphically continuous with and lithologically similar 

 to the mica-gneiss of Cecil county. If the Wissahickon gneiss and 

 the mica-gneiss of Cecil county are a unit, an inquiry into the struc- 

 tural relations and age of the Wissahickon gneiss becomes pertinent. 



The Peach Bottom slates, which appear to be a conformable mem- 

 ber of the series of which the Wissahickon mica-gneiss is an upper 

 member, have been referred by Professor James Hall to the 

 Calciferous. 



This determination was made on the basis of some rather dubious 

 fossils submitted to him by Persifor Prazer 2 and cannot, therefore, 



1 It is provisionally named the Wissahickon gneiss from the creek along the banks of 

 which it is finely exposed. 



2 Trans. Amer. Inst. Min. Eng., vol. xii, 1884, p. 358. 



