245 



The Oonglomerate bordering the Newton Loxver Falls 

 and Brighton Band of A:mygdaloid. — I have not observed 

 any isolated patches of conglomerate in the Newton Lower 

 Falls and Brighton band of amygdaloid, save in the detached 

 portion at the eastern end ; but this band is represented by so 

 few outcrops that many such islands might easily exist undis- 

 covered. The amygdaloid, however, is bordered by a con- 

 tinuous belt of conglomerate on the north similar to that on the 

 south, and these two are almost certainly connected in Brighton, 

 and were probably once continuous across the amygdaloid at 

 least as far west as the Charles River. The great overturned 

 anticlinal of which these two bands of conglomerate are merely 

 remnants, the roots, as it were, and the amygdaloid the de- 

 nuded axis, is completely broken down in Brighton by strike 

 faults, many of which have been attended by extravasation of 

 the amygdaloid. The structure of this district is very similar 

 to that of the Nantasket area, consisting of monoclinal dips, 

 faults, and igneous intrusions. The prevailing dip is northerly, 

 twenty to forty degrees ; and the principal dislocations are 

 along east-west lines, with the downthrow in each case on the 

 south, and so great that in several instances considerable masses 

 of the overlying slates have been dropped below the present 

 surface, and thus preserved from denudation. As may be 

 observed on the map, several of these slate patches are beyond 

 the borders of the amygdaloid, showing that some of the faults 

 failed to expose the la,tter rock ; and it seems fair to conclude 

 that there are probably still other faults not now marked by 

 either amygdaloid or late, and for that reasons undiscoverable. 



In this broad band of conglomerate, which is not less than 

 a mile wide, and has an average and nearly uniform northerly 

 dip of at least thirty degrees, we are able to see, by means of 

 the strips of slate and amygdaloid and the faults which they 

 represent, that the apparent thickness of the conglomerate, 

 about twenty-five hundred feet, must greatly exceed the true 

 thickness, perhaps several times ; and this adds immensely to 

 the probability of the conclusions I have stated in the preceding 



