214 R. PUAIPELLY SECULAR ROCK-DISINTEGRATION. 



gneiss that we had proved elsewhere to be separated from them by a 

 time-break. 



At many points there is complete structural conformity between the pre- 

 Cambrian granitoid gneiss and the overlying Cambrian. This is so as re- 

 gards the relation of the granitoid gneiss and overlying white gneiss and 

 conglomerate on Hoosac mountain. And it is even more marked on Clarks- 

 burg mountain. Here the quartzite, in describing a broadly-circling qua- 

 quaversal mantle around the end of the spur of granitoid gneiss, is crinkled 

 into minute fan-like plications, and the granitoid gneiss shows in fine lami- 

 nation the same plications in perfect parallelism, in horizontal section to 

 those of the quartzite, while there is equal parallelism between the axes of 

 the little crinkles in both rocks. This fine lamination disappears as we 

 recede from the contact a short distance into the granitoid gneiss area. It 

 is evident that this structure in the older rock was formed at the same time 

 and by the same pressure as that in the younger. 



This structural conformity, extending downward from elastics through 

 apparently transitional rocks into apparently much older crystalline rocks, 

 is a difficulty which is repeatedly encountered in the study of the areas of 

 crystalline schists. 



But I think the hypothesis of a pre-Cambrian decay of the granitoid gneiss 

 gives the key to the problem in the Green mountains. The Cambrian trans- 

 gression found the granitoid gneiss deeply disintegrated. As the breaching 

 line of the sea advanced landward, during the positive movement, the upper 

 zone of finer and wholly kaolinized material was removed to a distance, 

 while coarser material of the deeper zone of semi-kaolinization was deposited 

 nearer at hand, forming beds consisting chiefly of the little-altered feldspar 

 and quartz, with larger pebbles from the veins of quartz and of harder gran- 

 ites, as well as from occasional cores of partly disintegrated blocks. Beneath 

 these lay a zone in which the cohesion of the granite had been weakened by 

 the first stages of disintegration — the alteration of the mica or hornblendic 

 constituent lying between the other minerals. When the lateral thrust 

 which produced the folding took place, its effect was different in three posi- 

 tions on Hoosac mountain : on the eastern side, on the top of the arch, and in 

 the enfolded troughs on the western side. Along the monocline on the east, the 

 pressure found relief in a slipping movement, resulting in the production 

 of a laminated gneiss. On the western side, the material, resting on a base of 

 rigid gneiss, was thrown into minor folds and, being caught in the enfolding, 

 underwent a crushing action. On the other hand, at the top of the arch, as 

 suggested to me by Mr. Bailey Willis, being the point toward which the 

 movement of particles was directed, the material underwent only a crinkling 

 of its layers through the pressure acting toward the arch under the mass of 

 overlying rock. 



