STOP 4:  Outcrop of Wissahickon Schist with granite bands.
 
 
 
   Close inspection of the outcrop at the intersection of two trails reveals two rock types -- the mica-rich Wissahickon Schist and a white rock that forms bands paralleling the layering in the schist.  This white rock is a coarse-grained igneous rock, a granite with quartz, feldspar and muscovite.  In the granite, the minerals are not aligned with the layering, but are randomly oriented, often blocky in appearance and so do not appear to be deformed.  This is somewhat surprising since the bands of granite are quite contorted as they follow the layering of the schist.  Two explanations are possible.  1.  The granite has physical properties that limit its ability to deform internally.  If this were the case, then the majority of deformation occurs in the schist and the granite is a passive body shaped by changes around it.  While it is true that the granite is more competent than the schist and so harder to deform, folding should impart some fabric, especially where the folds are tight.  2.  The granite has intruded the schist, following pathways of least resistance within the rock.  These paths would follow layering in the schist.  The granite would, therefore, appear to be folded, but that is simply an artifact of the process of intrusion after deformation.  There is evidence in other outcrops that indicate that the granite is in fact younger than the deformation that affected the schist.

    Studies of the chemistry of the granite indicate that it is probably a product of melting of the schist.  A rock like the schist that is rich in muscovite and rich in a metamorphic fluid can melt at ≈ 600° C.  Other evidence from the minerals in the schist -- garnet, staurolite, kyanite, biotite, and muscovite -- also indicate that the schist reached temperatures near 600° C.  So the granite seen in the layers is thought to have come from a relatively nearby source.  In some areas, small to large bodies of granitic magma coalesced and solidified.  One of these can be seen on Forbidden Drive, just north of Bell's Mill Road.   

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