1875.] 415 [Dodge. 



metamorphism, however produced, must presumably have been at- 

 tended with more or less pressure, heat, expansion and softening, 

 directly produced by great depth of subsidence or other causes, and 

 assisted by the operation of these forces to increase each other. 

 The effect of pressure would, of course, be a tendency to escape in 

 the direction of least pressure, and fusion would be assisted by the 

 presence of alkalies. Furthermore, as some of the mineral constitu- 

 ents of the mass acted upon would be more affected than others, the 

 resulting outflows forced to the surface would have different compo- 

 sition from the less affected portions. Assuming heat as the soften- 

 ing cause, as hornblende is more fusible than felspar, the more truly 

 igneous rocks would, of course, a priori, be in great part hornblendic. 



As a matter of fact, so close is the resemblance in chemical compo- 

 sition, appearance and minerals developed in them, of the eruptives 

 among the slates and conglomerates, to the more fusible portions of 

 the crystallines, that it seems almost unreasonable to doubt that the 

 former were derived from among deep lying masses of the latter. 

 For the same reason, it is far from easy at the borders of the in- 

 cluded areas, to tell to which series such rocks belong, especially as 

 they are particularly abundant where the crystallines and stratified 

 rocks meet. In Weymouth, for instance, wherever the slates are 

 covered, and crystalline rock appears alone above the gravel, accu- 

 rate discrimination is almost impossible. 



The eruptives seem to follow one of two principal directions ; 

 E. N. E. — W. S. W. or N. W.— S. E.; as one or the other of the 

 chief axes of elevation has disturbed the rocks through whose cracks 

 they have been protruded. 



They vary somewhat in character. At Nahant and elsewhere, 

 there is a very coarse crystalline hornblendic rock, which is often 

 very micaceous. Another variety more common than the last is 

 crystalline granular. A mass of this at the corner of Elm and Mor- 

 rison Streets, Somerville, is strongly magnetic. Not infrequently 

 the joints of this kind of rock show glassy surfaces lined in one direc- 

 tion. A coarsely crystalline dolerite occurs in place on a hill 

 between Newton Corner and Charles River, near the road to Wa- 

 tertown. Specimens of this taken from a boulder at the western 

 end of Kendrick Street give a specific gravity of 2.97. The eruptive 

 masses near the corner of Milk and School Streets, Somerville, are 

 very varied, ranging from felspathic rock to hornblendic rock, often 

 very micaceous, sometimes granitic by the absence of hornblende. 



