26 Lovghlin — Relation* hetireen the Qui ncy Granite 



question. If they did, pebbles from the granite mass must 

 there too have formed only a local basal member of the Boston 

 Basin sediments, before the Quincy granite was protected from 

 erosion. The absence of such a basal member from the sur- 

 face to-day may be attributed to the formation of the bound- 

 ary fault, which left only the upper part of the conglomerate 

 exposed on the downthrow side. 



Significance of J 'a tilts. — Two principal systems of faults 

 are generally recognized through the formations involved in 

 the present discussion — one of nearly east-west, the other of 

 north-south trend. Four faults of the east-west system are of 



Fig. 2. ' 



Fig. 2. Map showing the Quincy granite mass, with the adjacent parts of 

 the Boston and Norfolk Basins, and the surrounding area of biotite granite 

 and feldsite. Only actual exposures are mapped within the two sedimen- 

 tary basins. Boundaries not definitely mentioned in the text are based on 

 Prof. Crosby's descriptions. (Op. cit.) Streams, roads, etc., are copied 

 from parts of the Boston, Boston Bay, Dedham, and Abington topographic 

 sheets of the U. S. Geol. Survey. 



especial importance in the present instance. Two are repre- 

 sented on the map (fig. 2), a third passes through Newton 

 Centre and Brookline in the middle of the Boston Basin, and 

 the fourth forms the north boundary of the Boston Basin. 

 These are interpreted as compression faults for the following 

 reasons: — The southernmost fault is almost entirely concealed 

 by drift; but, where exposed in the railroad cuts at Weymouth 

 Landing, the wall rocks, especially the Cambrian slates, are 

 highly slickensided and show the general effects of intense 

 compression and shearing. The fault plane is nearly or quite 



