216 R. PUxMPELLY — SECULAR ROCK-DISINTEGRATION. 



The actual plane of separation between the sedimentary elastics and the 

 purely cataclastic rocks is more difficult of definition, in the field, on the 

 monocline of the forceward side, because there the tendency both of the 

 shearing action of the slipping movement and of the action of granulation 

 upon the coarse feldspars is not only to obliterate the outlines of pebbles, 

 but to produce a uniform parallel foliation in which the dividing plane is lost. 



The instance mentioned on the southern bend of Clarksburg mountain 

 corresponds to the top of the arch, where the pebbles of the conglomerate, 

 though squeezed, are still distinct, and the plane dividing the two rocks is 

 not lost. Here I can imagine no other explanation for the fine lamination 

 in the granitoid gneiss (existing only near the contact with the quartzite 

 and crinkled in exact conformity with the quartzite lamination) than a super- 

 ficial weakening of the gneiss by partial disintegration. The fan-like crink- 

 ling belongs to the region of squeezing on the inner side of a fold. 



The transitions, such as I have described, from crystalline schists of clearly 

 clastic origin into others which are in structural conformity but must be 

 separated by a time-break, and which themselves pass downward into massive 

 granitoid rocks, may be thought to admit of two explanations. These 

 masked transitions occur along lines of great folding action, and are due to 

 the action of the folding force upon both the massive rock and the clastic 

 beds derived from its constituents. The alternatives are these : 



1. Either the granitic rock was massive and unaltered at the beginning of 

 the folding, and has undergone a shearing or squeezing action which produced 

 a schistose structure, only near the surface of the rock, without aflTecting it in 

 depth; *r 



2. The aflfected zone of the granite was already in such a condition of 

 weakness as caused it to act like the overlying clastic beds ; such a condition, 

 in fact, as exists in the lowest zone of disintegration where only the strength 

 of the micaceous constituent is aflfected. 



The validity of this second alternative is strongly indicated by the fact 

 that, in the transitions of this nature described above, the younger beds are 

 composed of detrital feldspar and blue quartz and rock pebbles, all derived 

 from the underlying granite and pointing toward a preexisting and deep 

 disintegration. 



Corroborative Evidence from the Southern Appalachians. — It seemed to me 

 doubtful whether, at the depth which then concealed the now exposed cores 

 of the Green mountain folds, the folding force was sufficient to overcome the 

 rigidity of the unaltered granite, except through the compensation of fault- 

 ing. In a visit, last spring, to the superb Doe river section in eastern Ten- 

 nessee, with Professor Van Hise, under the guidance of Mr. Bailey Willis, 

 we saw the Cambrian quartzite several thousand feet in thickness (its lower 

 beds abounding in coarse detrital feldspar and other fragmeutal minerals 



