28 Loughlin — Relations between the Quinei/ (i ranite 



or shallow son, was accompanied by gradual subsidence of the 

 region, and resulted in a great thickness of conglomerate and 

 sandstones. During the first stages of deposition, the local giant 

 conglomerate of the Norfolk Basin (and perhaps also of the 

 Boston Basin) was laid down, but the Quincy granite mass 

 was soon covered and protected from the erosion. The great 

 thickness of conglomerate in the Boston Basin, according to 



Fig. 3. 



Fig. 3. Hypothetical section showing the supposed relations between the 

 sediments of the Boston and Norfolk Basins before folding and faulting. 

 The red sandstone of the Norfolk Basin must, according to this view, have 

 been in general equivalent to the upper part of the conglomerate — " arkose " 

 series of the Boston Basin, the size of fragments diminishing and the amount 

 of red clay matrix increasing southward. The position of the gray sand- 

 stone of the Norfolk Basin, as stated in the text, is open to question. Prof. 

 Crosby's graben theory is generally applicable to the lower half of the sec- 

 tion, but, according to the writer's conclusions, the two basins must have 

 been continuous after the deposition of the basal strata. 



this hypothesis, is largely represented in the Norfolk Basin by 

 grit and reddish sandstone and slate, and implies that the prin- 

 cipal source of the sediments lay to the north and west. The 

 color of the red sandstones is due to a diminishing in size and 

 quantity of gray rock and mineral fragments, and to a corre- 

 sponding increase in the red clay which forms the matrix of 

 the conglomerate in both basins. Continued subsidence and 

 gradual change of climatic conditions caused the gradation 

 from conglomerate through sandstone and slate, and finally 

 into the blue-black Cambridge slate. How far southward the 

 latter extended is questionable, but its thickness, possibly 2000 

 ft. or more,* in the Boston Basin is sufficient to warrant the 

 supposition that it formerly extended over the sediments of the 

 Norfolk Basin also. 



Following this period of deposition came one of north-south 

 tangential compression. The mass of newly formed sediments 

 offered the least resistance to deformation and, yielding between 

 the blocks of rigid biotite granite, were resolved into a series of 



* L. LaForge, quoted by J. E. Mansfield, op. cit., p. 196. 



