GENERAL STRUCTURE AND CORRELATION. 



31 



deepened the water shoreward, the calcareous materials accumulated above 

 the earlier detrital beds, so that we may imagine that, while the later 

 beds of the Cambrian were being made of sand and gravel in shallow 

 water, the lower beds of the great limestone formation were being deposited 

 offshore. Later, with a change of some kind in the conditions, there came 

 the deposit of finer material over the previously shallow region, while the 

 accumulation of limestone, with Lower Silurian organisms, still continued 

 offshore. Still later, by another change in the conditions, the deposit of 

 finer detrital material extended far to seaward, covering everywhere the 

 limestone accumulations. 



jr. 



w/nf* Congiom Coarse 





Stan/brti TnafiszlianciL 

 Gneiss Gneis s 



Fig. 8. — Map showing the varying character of the Cambrian rock* in con- 

 tact with the pro-Cambrian granitoid gneiss mass on Hoosac mountain. 



As we are not yet able to say to what depth into the Cambrian the lime- 

 stone may extend in the Hoosac valley, so, also, we are unable to sav to 

 what extent the lower beds of schists on Hoosac mountain may represent 

 Cambrian time. 



Mr. Wolff has shown that the Cambrian quartzite horizon, which is a 

 true conglomerate on the top of the arch at the north end of the granitoid 

 gneiss area, consists on the eastern and easterly dipping limb of coarse 

 gneisses, showing only occasional pebbles, as in the tunnel, while on the 

 western and crumpled limb it is represented by finer-grained white gneiss. 

 These relations are shown in Fig. 8. We may suppose an island of 

 coarse granitoid gneiss with a disintegrated mantle, and imagine this latter 

 to have been abraded down to its less disintegrated zone, and the resulting 



