199 



We have now a clue to the structure of this entire area of 

 amygdaloid and conglomerate. Originally the surface was en- 

 tirely composed of the former rock ; and over this Vv^as deposited 

 by the Primordial sea a continuous sheet of conglomerate, with 

 sandstone and slate toward the top. The Primordial beds 

 were first tilted so as to have a nearly uniform southerly dip of 

 10^-20°, and then broken by numerous transverse and strike 

 faults, the latter predominating and having the downthrow 

 apparently always to the north. In some cases the faults are 

 traceable for considerable distances, nearly a mile, and are 

 marked by a downthrow of hundreds of feet; while others are 

 short, with a dislocation of tens of feet or even less. Very 

 often, as in the examples described, the faults are accompanied 

 by dykes of diorite ; and as these, by reason of their jointing, 

 are more easily removed by the agents of erosion than the tough 

 amygdaloid and indurated conglomerate, there results the curi- 

 ous checker-board topography noticed by many observers ; the 

 surface being, to a considerable extent, divided by narrow de- 

 files into square or rectangular hills with precipitous slopes. 

 In some cases, however, the cliffs seem to be the product of the 

 dislocations alone. 



Although the amygdaloid is undoubtedly exposed 'mainly 

 through the agency of faults, yet some of the masses of this 

 rock are too irregular and tortuous to be accounted for in this 

 way, and it appears necessary to regard them as exotic. In 

 the second ledge noticed above, where the amygdaloid under- 

 lies, with some appearance of conformability, strata which are 

 almost certainly not the base of the conglomerate, it probably 

 forms an intrusive bed. Another example, precisely similar, 

 occurs a few rods farther south, at the north-east corner of the 

 large hill on which the Atlantic House stands. Here a^ small 

 mass of beds identical with those already described as abutting 

 against the north base of this hill on the same line of strike, 

 but farther west, — gray and greenish sandstone interstratified 

 with conglomerate holding abundant pebbles of amygdaloid, — 

 is not only cut off by the amygdaloid in the direction of the 



