14 GKEEX MOUNTAINS IN MASSACHUSETTS. 



tinuity of deposition from the quartzite upward in each column, and we 

 have also petrographic identity in the schists of the two columns. 



Prof. Emmons attempted to explain the similarity of the Greylock 

 schists to those of Iloosac mountain by deriving the supposedly younger 

 Grreylock beds from the destruction of the supposedly older Hoosac rocks, 

 but Mr. Wolff finds, under the microscope, not only no evidence to sup- 

 port the idea of such derivation for the Grreylock schists, but that the 

 principal constituent minerals of these schists were in each column all 

 crystallized in place. Early in the course of the work it was proved that 

 the limestone was not present as such in the Hoosac column. But at 

 two points near Cheshire harbor, and east of North Adams, we found schist 

 outliers extending out from the Hoosac column, and at the extreme western 

 ends conformably related to the great limestone; in one case occupying a 

 synclinal trough in it, and in the other either capping it or interbedded in it, 



Almost at the beginning of the survey, although we had as yet none 

 of the proofs above given as to the equivalence of the valley quartzite with 

 the Hoosac conglomerate and white gneiss, the strong possibility that at 

 least apart of the Grreylock column was contemporaneous with a part of the 

 Hoosac column had presented itself to me. This possibility was strength- 

 ened when we had correlated the quartzite with the white gneiss and con- 

 glomerate beds as equivalents. The truth of this hypothesis could be tested 

 only by finding beds showing lateral transition to bridge the narrow belt 

 between the Stockbridge limestone and the Hoosac schist. 



In the progress of our survey we found, at various points between the 

 valley and the mountain, and always east of the limestone, outci'ops of a, 

 peculiar rotten schist — quartz and mica with some feldspar, with the mica 

 arranged in long narrow flakes and with sufficient calcite to show the cause 

 of the decomposition. The occurrence of this peculiar calcareous rock 

 along the boundary between limestone and quartzite, as on Tophet creek 

 and below the albitic schist in the western end of the tunnel, shows that it 

 belongs in the horizon of the vertical transition between the quartzite ami 

 the limestone, and it seems to represent also the lateral transition zone in 

 this horizon between the Iloosac and Greylock columns. 



East of North Adams, on the road to Briggsville, the river cuts longitu- 



