60 bulletin: museum of comparative zoology. 



area which arc beneath the melaphyr. These would be igneous in any 

 case. All of the known contacts, so far as they yield evidence, point 

 toward the intrusive nature of the igneous rock. 



3. Evidence from Structural Relations. — On Professor Crosby's 

 map of the Boston area (The Geology of Eastern Massachusetts), the 

 melaphyr is made to consist of two long arms, extending in a generally 

 east and west direction and united in a rather broad area west of the 

 Charles River. Each of these arms is bordered by conglomerate, and 

 this, in turn, is rimmed with slate. Between the arms the conglomerate 

 occupies the greater portion of the area, but is parted by a wedge of slate 

 which broadens eastward till it encircles the two conglomerate masses, 

 and unites with the slate areas to the north and south. This distribu- 

 tion would seem to lend striking support to the idea that the melaphyr 

 is at the bottom of the series and is followed by the mass of the con- 

 glomerate with the slate at the very top. There are indicated two well- 

 defined anticlines with melaphyr exposed in the axes and wrapping about 

 the spoon-shaped end of the synclinal area of slate and conglomerate 

 (see Crosby, '89, pp. 9-12 a ). 



It may be assumed for the moment that this interpretation of the 

 general structure is correct. It then becomes necessary to ascertain 

 whether or not the distribution of the melaphyr accords with this struc- 

 ture. In Brookline the melaphyr occupies the middle of the southern 

 anticline. It does not, however, appear to lie symmetrically about the 

 felsite axis exposed in the southern part of Newton. In the consider- 

 able space between Brookline and Greenwood Streets the melaphyr does 

 not appear. Although it is possible that it does underlie a portion of 

 this area, the occurrence of five well-marked outcrops of conglomerate 

 and the presence of great numbers of large conglomerate boulders makes 

 it probable that the latter is the underlying rock. The conglomerate 

 found on the southern side of Greenwood Street is made up almost 

 wholly of felsitic debris. It closely resembles a conglomerate appearing 

 near the corner of Dedham and Walnut Streets. The strike of the 

 Greenwood Street conglomerate would carry it very near to the Dedham 

 Street locality. It is not impossible that they are continuous beneath the 

 intervening space. It is not at all certain that the melaphyr occurs be- 



1 The position of these anticlinal areas is indicated, on the accompanying map 

 (Plate 2), hy heavy black lines (d d' and e e'). These are drawn, as nearly as pos- 

 sible, through the middle portions of the melaphyr areas as expressed on Crosby's 

 map. A heavy broken line (if) is drawn through the middle of the synclinal area 

 of slate. 





