276 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 39 



faces, called slichensides. It is also indicated by the finely powdered 

 crushed rock (gouge) now represented by clay. This faulted struc- 

 ture is sketched on the label just within the protecting screen (pi. 7), 

 where the fault plane is shown cutting across several of the layers 

 producing a fault zone along which water could percolate freely and 

 minerals might be deposited. Other features related to faults may be 

 studied here, such as the dip^ the angle between the fault surface and 

 the horizontal plane, the hanging waU., the side that overhangs in an 

 inclined fault, the footwall^ the other side, the heave^ the horizontal 

 displacement of the rocks, and the throw^ the vertical displacement. 

 Proceeding down Adams Mill Road (pi. 8) a rock cut along the 

 east side near its junction with Harvard Street (fig. 1, ^) shows 

 a considerable thickness of metamorphic rock, folded schists, on each 

 side of a mass of the igneous rock, granite, capped at the near end 

 by horizontal sandy sedimentary layers. These outcrops, the surface 

 extension of the bear pit exposures in the park below, embrace all 

 three classes of rocks, besides exposing an instructive geologic 

 structure section. The granite is an instrusive rock, since it shows 

 it had been thrust from below into the schist, and is also a crystalline 

 rock, composed of the constituent pHmary minerals, quartz and feld- 

 spar, and accessory minerals, mica and sometimes black hornblende, 

 in crystal form. As now exposed the fair-sized granite joint blocks 

 indicate their intrusion into the metamorphic schists by the bending 

 of the latter into an anticline. A master joint block made up of 

 many small blocks may be noted in this general area as well as cases 

 of the metamorphosed granite changed into granite gneiss. Wliether 

 these schists are of igneous or sedimentary origin is still a debated 

 question, but the fact that some of the layers contain small quartz 

 pebbles, apparently of stream origin, seems to indicate that at least 

 certain parts of the schist were originally mud deposits bearing 

 water-worn pebbles. Considered as sedimentary beds these layers 

 show a definite dip, registered by the angle of entrance in the surface, 

 and strike, the direction of surface outcrop of any particular layer. 

 The top of this anticline plainly has been worn away and the general 

 surface of the old rocks is undulating. Prolonged weathering by 

 water solution and the absorption of water by hydration has reduced 

 most of the originally hard granite and schist to material so soft 

 that the finger can easily penetrate it, although the original form 

 and structure of both the igneous and metamorphic rocks are still 

 evident. Many of the rocks, both hard and soft, are stained brown 

 to red, showing oxidation of the small amount of iron in them. The 

 gradation from hard rock at the street level up through the over- 

 lying disintegrated subsoil to soil at the top of the outcrop called 

 residual soil because formed in place, is also evident. These together 



