ORDOVICIAN ROCKS 305 



parallel structures and inclined 60 to 70 degrees southeast (see plate 53). 

 Anticlines and synclines alternate. The strike is north 50 to 80 degrees 

 east with a varying pitch of 5 to 25 degrees. 



There are many abandoned quarries in the mica-gneiss, and large 

 quarries are in active operation between Chestnut hill and Germantown 

 and on Cobbs creek west of Haverford. Many country town homes are 

 built of this material. It has been used in college halls at Bryn Mawr, 

 Princeton, Williams, and Vassar. 



Thickness of the mica-gneiss. — It is not possible to determine with any 

 exactness the thickness of the mica-gneiss. The isoclinal folding and 

 constant variation in beds give a false idea of its thickness, which proba- 

 bly lies between 1,000 and 2,000 feet. 



Name. — The Wissahickon creek affords an excellent section across the 

 strike of the formation. Because the mica-gneiss is so finely exposed in 

 the gorge of this stream it is called the Wissahickon mica-gneiss (see 

 plate 54). 



Stratigraphic relations. — The determination of the age of the mica-gneiss 

 rests wholly for its evidence on the stratigraphic relations which the 

 formation sustains with fossil-bearing sediments. The mica gneiss con- 

 tains within itself no clue to its age. 



In northern Delaware and in the southwestern part of Chester county, 

 Pennsylvania, limestone is exposed in several abandoned quarries. Crys- 

 talline limestone is brought to the surface b}^ means of anticlinal struct- 

 ures and bears on the limbs of the anticlines conformable beds of mica- 

 gneiss similar to the Wissahickon mica-gneiss. This relation is also 

 shown in several limestone outcrops west of West Chester, Pennsylvania. 

 In the latter localities the limestone is in line with the strike of the 

 Chester Valley limestone, and is without doubt stratigraphically con- 

 tinuous with that formation beneath the mica-schist. There is no evi- 

 dence, except very locall}', of thrust movement between limestone and 

 mica-gneiss. The number and extent of such occurrences of gneiss and 

 limestone, and the large area involved, renders an overturned structure 

 improbable. Presumptively the mica-gneiss lies above the limestone. 

 Not only does the mica-gneiss bear this stratigraphic relation to presum- 

 ably Cambro-Ordovician limestone, but, as has been pointed out, it grades 

 across the strike into the Ordovician mica-schist just described. This 

 lithologic gradation and stratigraphic continuity can be seen in sections 

 furnished by Brandywine creek and Buck run. 



It has been affirmed by G. H. Williams* that the eruptive material 

 is confined to the eastern belt of gneiss and is not found in the recog- 



• "The petrography and structure of the Piedmont plateau in Maryland.' 1 Bull. Geol. Soc. Am. 

 vol. 2, 1891, p. 3IG. 



XLI-Bum,. Geoi.. Soc. Am., Vol. 10. 1904 



