and the Adjacent Sedimentary Formations. 27 



vertical, but, as a whole, dips steeply to the south with granite 

 as the hanging wall.* In the Norfolk Basin, as already men- 

 tioned, the strata lie nearly or quite vertical. Furthermore, 

 slaty cleavage, minor faults, and the more or less conspicuous 

 development of secondary mica, all bear witness to intense 

 compression. In a compressed area consisting of granite and 

 fine sediments, the great contrast in resistance to stress would 

 promote a thrust fault between the two. 



The fault between the Quincy granite and the Boston Basin 

 sediments has already been described as of a reversed charac- 

 ter. Shearing effects are excellently shown in the " arkose " 

 by minor vein-filled joints, and by the cleavage in slate part- 

 ings, both diagonal to the dip of the beds and to the trend of 

 the fault. These facts, together with the semi-metamorphic 

 character of the " arkose," favor the interpretation of the fault 

 as one of compression. 



The Newton Centre fault needs no discussion. Exposures 

 of it show the slate overridden by conglomerate in an over- 

 turned syncline. 



The northern boundary fault of the Boston Basin is com- 

 pletely concealed and evidence regarding it cannot be satisfac- 

 tory ; but a small area of conglomerate and sandstone in 

 Medford shows the effects of severe compression, and the large 

 Medford diabase dike crosses the fault without sending branches 

 along it. There must, therefore, have been intense compres- 

 sion along the fault, and the fault must have remained tightly 

 sealed after its formation. There exists, also, as along the 

 southernmost of the faults, the contact between rigid granite 

 and the more flexible sediments. As these faults all lie nearly 

 parallel to the axes of the folds, it seems proper to attribute both 

 faults and folds to a single period of compression. Movement 

 along these faults may have been accompanied by some rota- 

 tion of the fault blocks, as will be shown beyond. It is also pos- 

 sible that, both before and after the compression period, nor- 

 mal faulting may have taken place along some of these planes, 

 but not to such an extent as to obscure the effects of the com- 

 pression faulting. 



Geologic History. — A restoration of original conditions 

 which accord with the present structure is represented in fig. 

 3. The Quincy granite mass, with only its top exposed to ero- 

 sion through a cover of Cambrian slates, then formed a rela- 

 tively low prominence within a lowland or shallow sea, floored 

 wholly or in part with Cambrian slate of moderate dip. The 

 surrounding highlands were made up of the biotite-granite 

 series, older felsites, and remnants of Cambrian strata. 

 Deposition of detritus from the highlands, and in the lowland, 

 * W. O. Crosby, op. cit., p. 392, et seq. 



