and the Adjacent Sedimentary Formations. 



29 



folds. The southernmost fold in question, a close syncline 

 (simple or, perhaps, compound) gave way under excessive strain 

 to a compression fault (tig. 4). Similarly the syncline forming 

 on the north side of the rigid Quincy granite mass gave way to 

 a compression fault, the rigid granite luding up against the con- 

 glomerate and " arkose." The vertical dip of the giant con- 

 glomerate close to the Quincy granite mass indicates that the 



&&■ 





Fig. 4. 



'£>W 



*>&/<. 



fied 



0" 



So"' 



dstonQ 



VM 







incy x &re 



Fig. 4. N-S section through Bear Hill (see fig. 2) showing relation 

 between the sediments of the Boston and Norfolk Basins. The compound 

 synclinal structure of the Norfolk Basin, though very possible, is hypotheti- 

 cal, and serves to indicate the amount of compression expressed in outcrop. 

 An equally good interpretation, based upon the attitudes of outcrops, 

 would be a simple close synclinal fold with the south limb removed by 

 faulting. The "arkose" of the Boston Basin is exposed only along the 

 boundary fault. Its total extent is not known. Vertical scale about twice 

 the horizontal. 



fault block (or blocks) containing the present exposed mass of 

 Quincy granite must have suffered some rotation during fault- 

 ing. This rotation, as already stated, accounts for the position 

 of the still uneroded portion of the porphyry contact zone. 

 According to Prof. Crosby's interpretation, the Quincy gran- 

 ite mass was a tilted fault block prior to the period of folding 

 and compression faulting. Such earlier tilting may have been 

 increased by the later rotation. 



On the other side of the compressed area, the granite was 

 thrust from the north against the sediments to form the north- 

 ernmost of the four faults. The Newton Centre fault resulted 

 in the breaking of an overturned syncline, relatively massive 

 conglomerate overriding relatively plastic slate from the north. 

 Other minor compression faults were formed, and some of 

 them have been mentioned ; but they are inconspicuous beside 

 the four just considered. 



These movements were accompanied by the development of 

 secondary minerals, depending upon the composition of the 

 rock. Quartz and sericite are the most common minerals in 



