330 G. p. MERRILL — DISINTEGRATION OF GRANITIC ROCKS. 



Aside from these, an abundant sprinkling of well rounded pebbles of 

 from one to 5 or 6 pounds weight each form one of the most character- 

 istic features of the gravels. It seems impossible that such material 

 could have been brought to its present position except by the aid of 

 currents or wave-action so energetic as to erode the then existing decom- 

 posed granitic material which the lithologic character of the Potomac 

 formation, as above given, tends to prove existed. 



The point which I now wish to make is, however, that all such ma- 

 terial was removed from its position in situ prior to the deposition of 

 these gravels. The fact that everywhere along the lower part of the de- 

 posits there is a notable lack of the angular quartz fragments and jointed 

 blocks of granite such as now form so conspicuous a feature leads, as it 

 seems to me, irresistibly to the conclusion that prior to their deposition 

 all loose and partially decomposed matter was eroded away and the later 

 deposition made upon hard and comparatively fresh surfaces. Hence 

 the disintegration as we now find it, extending in some cases to a depth 

 of 50 or more feet, is almost wholly post-Cretaceous. 



That this apparently rapid rate of decomposition is not anomalous is 

 well illustrated in a large dike of diabase at Medford, Massachusetts, the 

 petrographic nature of which has been made known by Dr Hobbs.* 

 Portions of this dike are in an advanced stage of disintegration, which is 

 undoubtedly postglacial. The writer hopes todescribe the changes which 

 have here taken place in another paper. 



As a matter of passing interest and as bearing upon the same general 

 subject, I may mention the fact that the pebbles of felsitic rock noted as 

 occurring in the Potomac gravels are, as a rule, in a condition of such 

 complete decomposition (kaolinization) as to fall to pieces except when 

 handled with the greatest care. Indeed, wherever exposed through the 

 cutting of streets, they fall away quickly to loose sand. Nevertheless, 

 the outlines of these pebbles are sharply oval and the surfaces smooth 

 and almost polished. They are beyond question water-worn pebbles, 

 and as such could only have assumed their rounded form when their 

 materials were in an entirely fresh and undecomposed condition — that 

 is to say, their decomposition was posterior to their deposition, or at least 

 to the time of their becoming water-worn. 



This particular occurrence I regard of interest as showing, first, the 

 great depth to which disintegration can be carried without excessive 

 decomposition, and, secondly, the relative rapidit}^ of the process. I 

 should add that in areas examined farther to the west and south, beyond 

 the limit of the Cretaceous submergence, I find similar rocks in a state 



* Bull. Museum of Comparative Zoology, vol. xvi, no. 1, 1888. 



