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! 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 
River 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. 
