the Middle Atlantic Slope. 139 



again about the Schuylkill, once more decreasing in size in the 

 outliers lodged behind Chestnut hill and Edge hill, and again 

 assuming considerable dimensions on the Delaware ; but in 

 this part of the area of the formation the quartzite pebbles 

 occur on the smaller streams and over the divides as well as on 

 the great rivers. Finally, in the Sand Hills quartzite is nearly 

 or entirely absent. In short, the quartzite pebbles occur only in 

 the vicinity of those rivers which flow through the quartzites of 

 the Appalachian and upper Piedmont regions, and their abun- 

 dance and size vary with the dimensions of the rivers and the 

 proximity of the ridges. 



The composition of the pebbles is equally significant : Those 

 found about Washington and Richmond sometimes exhibit 

 Scoliihus borings and casts of brachiopods, etc. , identical with 

 those found in situ in the axial quartzites of the Blue Ridge 

 and adjacent Appalachian ranges in their less metamorphosed 

 portions ; the pink quartzite of the pebbles found over the 

 Schuylkill-Delaware divide is in all respects similar to that of 

 certain exceptionally obdurate ledges in the quartzitic axes of 

 the low ranges here encroaching upon the Piedmont region ; 

 and the distinctive pebbles are in both these as well as in other 

 cases confined either to the vicinity of the distinctive ledges or 

 to the lower reaches of the rivers transecting them. 



So by distribution' and composition, the q uartzite pebbles of 

 the Potomac formation may be traced to their parent ledges in 

 the Appalachians ; and their distribution indicates at the same 

 time the ancient river-courses and the shore lines along which 

 they were transported. 



2. The quartz of the second class of pebbles (which some- 

 times assume the dimensions of respectable bowlders) is petro- 

 graphically identical with the vein quartz everywhere inter- 

 secting the Piedmont crystallines ; the pebbles, like those of 

 quartzite, are generally largest in the greater valleys and to- 

 ward the western margin of the formation, though considerable 

 bowlders occasionally occur in all parts of it.; they are most 

 abundant where the quartz veins are large and numerous ; and 

 local peculiarities in the vein quartz are reflected in the lee- 

 ward pebbles. It is evident, in short, that the pebbles are de- 

 rived from adjacent veins. Indeed in a section in the north- 

 western part of Washington a train of fragments of a quartz 

 vein intersecting the Archean gneiss and projecting into the 

 superjacent Potomac gravels, is traceable in the gravels for 

 some distance before the fragments so far lose their angles as 

 to become undistinguishable from the neighboring erratic 

 pebbles. 



3. The arkose is made up of angular grains of quartz, crys- 

 tals of feldspar or flakes of kaolin, scales of mica, etc., the 



