254 



the contortions are connected genetically with the great 

 strike-fault supposed , to separate the slate and melaphyr, 

 we can only conjecture. 



The extreme northwest corner of this area is low and devoid 

 of outcrops ; but the ledges on the south and east are evidently 

 suthcient to justify mapping the whole as slate. Eastward, on 

 the contrary, the ledges, alike of melaphyr, slate, and diabase, 

 end at the western edge of the meadow and its barrier beach ; 

 and the band of slate represented as crossing the meadow and 

 the northern end of Pleasant Hill is based largely upon theoretic 

 considerations. In the way of direct evidence we haA^e the 

 important fact that this north shore, including the beach and 

 the section of till above it, is made up almost wholly of debris 

 of the gray slate, many of the fragments being large and 

 ano-ular. There are, of course, occasional bowlders of granite 

 and felsitc, glacial erratics from the hills north of Boston 

 Harbor; and more rarely, also, a stray block of conglomerate 

 or sandstone. But it is perfectly obvious that we find here no 

 adequate representation of the great development of the con- 

 glomerate series — conglomerate, sandstone, and red slate — 

 outcropping immediately south of this shore and striking 

 directly toward it. Considering the extremely local origin of 

 the main part of the drift, we have no alternative but to 

 conclude that this shore is underlain by tlie gray slate and not 

 by either the conglomerate series or the melaphyr. This pro- 

 longation of the slate series directly across the strike of the 

 conglomerate series implies, of course, an eastward extension 

 of the fault between the slate and melaphyr. At the northeast 

 corner of the mela})hyr, as ah-eady explained, this fault should 

 be carried a little nearer the shore than the map shows it ; Imt 

 there is, on the whole, no theoretical part of the map which I 

 am able to regard with greater satisfaction. According to this 

 view, the Melville Garden and Planter's Fields area of the 

 couiilomeratc series is .l)ounded on the west bv a profound 

 upthrow fault, and on the nortli bv an equally profound 

 downthrow fault. 



