GENERAL STRUCTURE AND CORRELATION. 21 



overfblds, but with the axes striking northwest to southeast ; while still 

 farther westward they are overtoiled to the west, but with the axes in the 

 normal position of the Green mountain folds — nearly north and south. 



Looking at the map and sections of Greylock, Pis. i, xvm, xxm, we find 

 a great basin-bottomed mass, thrown into numerous more or less overturned 

 folds, with axes in the normal Green mountain position, and inclined from 

 each end toward the middle. Again, if we look at the eastern border of the 

 map, we find in the observed strikes and dips of the conglomerate gneiss and 

 schist east of the granitoid, no trace of a departure from the general Green 

 mountain direction. 



This local modification in the structure of Hoosac mountain must be 

 due to some local cause, whirl) I think must be sought in the pre-Cambrian 

 topography. The Greylock basin of sediment was guarded on the north 

 by the large mass of granitoid gneiss of Clarksburg mountain, and on the 

 south by the great body of pre-Cambrian rocks which are now masked by 

 the Dalton and Windsor quartzite. I imagine that the lateral thrust to 

 which the foldings are due met with greater resistance opposite these more 

 rigid granitic masses than in the interval, and that the abnormal overfoldings 

 to the south, described above, are the result of compensatory movement. 

 The Hoosac mountain cross sections show a much more marked overturn 

 than is observed to either the east or west of it. The axial plane of the 

 principal overturned fold on the west side of the mountain lies very flat. 

 We may suppose the greater rigidity of the granitoid gneiss to have 

 caused it to yield as a unit to the contracting force. Only its relatively 

 narrow top participated in the actual folding and was carried over to form, 

 with the leeward, protected beds, a flat-lying, compressed syncline. 



A similar overturn, though not so flat, was observed by us on Sumner 

 mountain, in Pownal, on the west of the Clarksburg mass of granitoid 

 gneiss. Section a on Plate in was made by Mr. B. T. Putnam. I have 

 added my interpretation in dotted lines. This outlier is separated from 

 Clarksburg mountain by Broad brook, this interval being occupied by 

 the quartzite. The large Clarksburg mass of granitoid gneiss remained 

 a dome mantled by the Cambrian quartzite, and showing the effect of the 

 folding force only in the induced lamination common to itself and the 



