FOLDING CONFINED TO CLASTIC ROCKS. 217 



recognizable as comincr from the underlying granite) resting on the granite, 

 with the contact plane dipping 65° northwestward under the quartzite. To 

 the west of the contact the whole immense thickness of the quartzite is visi- 

 bly folded back upon itself. That the granite took no part in all this fold- 

 ing is evident, as pointed out by Mr. Willis, from the fact that numerous 

 basic dikes which traverse it, without cutting the quartzite, have retained 

 their shape in unwarped planes. Any compensating movements that affected 

 this granite during the folding of the quartzite must have been of the nature 

 of faultiugs. 



Influence of antecedent Disintegration on Kock Formation. 



Formation of basal Beds. — I am disposed to regard all the basal conglom- 

 erates and sandstones or quartzites that mark the beginnings of geological 

 periods, as having been produced during positive movements by the breach- 

 ing action of the ocean advancing over previously deeply disintegrated land 

 surfaces. But, in the absence of more extended study of the facts in the 

 field, I have confined my conclusions wholly to inferences drawn from those 

 elastics in which detrital feldspars, with or without rock pebbles, play an 

 important part. It would be wrong to assert that all of the detrital mate- 

 rials forming such basal clastic beds were derived only from a disintegrated 

 mantle ; for any geologist who has observed the breaching action of the ocean 

 on a cliff-bound coast, will recognize the importance ascribed, particularly by 

 von Richthofen,* to this greatest of all abrading forces. 



Undoubtedly, under certain relations between minimum rapidity of the 

 positive movement and maximum steepness of the seaward slope of the land, 

 minimum hardness of the rock and minimum depth of disintegration, the 

 breaching action would work to a greater or less depth into the solid rock, 

 establishing in places its resulting surface (Ramsay's "plane of marine de- 

 nudation ; " von Richthofeu's " plane of abrasioft ") below the region of 

 secular disintegration. 



But these conditions do not appear to have been general in the Appa- 

 lachians during the Cambrian transgression. This is shown on Clarksburg 

 mountain by the fact that the breaching did not extend so deep as the decay 

 in the Stamford dike. It is shown still more clearly, I think, by the abun- 

 dance of coarse detrital feldspar in the basal beds of the Cambrian. It seems 

 to me that the same evidence is furnished by the blue quartz conglomerate ; 

 for the pebbles of this mineral are derived from the quartz of the coarse 

 granite and gneiss. 



Formation of detrital Bocks. — The breaking down of cliffs of solid granite 

 or gneiss would produce blocks which would grind down to fine flour of at- 



*Ghiua, vol. II, 1882, p. 707. 

 XXXIII— Bull. Geol. Soc. Am., Vol. 2, 1890. 



