54 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



The traps have the form of fairly regular dikes and sills. Even the 

 earliest of these gives evidence of having followed joint cracks more or 

 less perfectly developed. The melaphyr, on the other hand, where it 

 has come into igneous contact with the sediments, has torn its way 

 through without regard for their structures. It cannot be doubted 

 that portions of the sediments, at least, were but slightly consolidated 

 at the time that the igneous mass was forced into them. The melaphyr 

 shows evidence of having undergone nearly the same structural changes 

 as the rocks with which it is associated. It is cleaved, in many places, 

 quite as perfectly as the slates and conglomerates. It is cut by the 

 same joint systems. There is evidence for believing it to have been 

 tilted to somewhat the same extent as the associated sediments. 



The melaphyr bears many inclusions of felsite x and of slate. In 

 many cases these appear to have been partially fused, sometimes to the 

 point of becoming intimately mixed with the melaphyr itself. These 

 inclusions are highly characteristic of this rock. 



Melaphyr occurs, within the limits of the so-called Boston Basin, at 

 Nantasket, in Hingham, on Hough's Neck, Quincy, on the Neponset 

 Biver south of Mattapan, at Needham, and in Brookline, Newton, and 

 Brighton. It is with the latter area that this paper deals. 



The melaphyr, although so much altered internally, is yet a fairly 

 resistant rock. It usually stands out in prominent ridges and is thus 

 readily traced. If it were known to be associated with a definite 

 horizon, it would be extremely useful in deciphering the very involved 

 structure of the region. Professor Crosby ('89, p. 10), and others who 

 have worked with him have regarded these rocks as contemporaneous 

 flows, occurring at or near the base of the conglomerate series. If such 

 were the case, the melaphyr would have a value as an horizon marker. 

 The evidence in the Brookline, Newton, and Brighton areas seems to 

 indicate that the melaphyr is intrusive into the conglomerate and even 

 into the overlying slate. If such is the case, it is not associated with a 

 definite horizon and thus loses the greater part of its value as a guide to 

 the structure. 



The evidence to be presented is of three kinds : (1) The sedi- 

 ments which appear to overlie the melaphyr do not contain fragments of 

 it ; (2) The contacts, wherever observed, are of igneous character ; 

 (3) The melaphyr masses do not show that structural accordance with 

 the sediments which would be demanded of them as flows. 



1 The ancient rhyolitic rocks of this region are known, locally, as felsites. It is 

 in this sense that the term is used in this paper. 



