136 STONE IMPLEMENTS [etu.ann.is 



more carefully we find that nature has not provided any other form of 

 the several toiij^h varieties of stoue so jierfectly suited to the purposes 

 of the stoue-iuiplenient flaker as the bowlder or pebble. 



Each river brought down I'roui the highland only such varieties of 

 stone as belonged to the drainage of tliat river, so that in one valley 

 one set of materials prevails and in another a ditt'erent set of materials 

 appears, varying with the geologic formations of the region drained. 

 Kivcrs having identical formations have nearly identical bowlders-, 

 long rivers crossing numerous foiinations liave many varieties; short 

 rivers crossing but few formations have but a limited number. 



There is also a selection as to size by each drainage way. Xear the 

 base of the highland, where the force of the current is reduced by meet- 

 ing tidewater, the larger bowlders are drojjped, the smaller ones are 

 (U'posited farther down, and the pebbles and sand are carried far sea- 

 ward. Small and weak streams transport fewer ])ieces and droj) them 

 sooner. This selection does not hold good with ice transportation, 

 which agency has carried irregular masses of stone to many widely 

 distributed points. Notwithstanding the fact that all water-transported 

 stones are more or less rounded, there is a selection with respect to 

 degree of roundness. If dropped earlj- in the progress of transporta- 

 tion, the bowlder is imperfectly rounded; if carried far, it is fully 

 rounded. Near the margin of the highland, therefore, there is a large 

 percentage of imperfectly rounded stones, and farther out there is a 

 small percentage of decidedly irregular forms. These conditions are 

 probably considerably modified by the action of the waves along the 

 ancient seashore which skirted the base of the highland. Sucii frag- 

 ments as were subjected to wave action became fully rounded and were 

 deposited in beds along the ancient beach-lines. It is not easy to dis- 

 tinguish the beach-rolled material from that rounded by the llow of 

 streams, both agencies having no doubt frequently acted in turn on 

 the same material. 



Again, we observe that on river banks near the base of the highland 

 many varieties of rock are present, but with each mile as we descend 

 the number is diminished — the softer species are reduced to sand as 

 they move toward the sea and one after another disappears. Quartz, 

 being the hardest, is last to yield to the erosive agents, and at various 

 points along the ocean beach well-polished quartz jjebbles are found. 



A comparison of the Potomac and Patuxent rivers with respect to 

 these points is instructive. In ancient times both streams, as they 

 descended from the mountains, gathered fragments of rock and carried 

 them downward until the soft and friable ones were reduced to sand 

 and the tough, flinty varieties became bowlders and pebbles. The 

 latter consisted chiefly of (piartz and quartzite. The Potomac was a 

 long stream, heading far in the west and cutting through many ranges 

 of mountains and hills. It crossed heavy beds of quartzite in the 

 region of the Blue ridge. This rock is tough and massive, and breaks 



