MUSEUM OF COMrAKATIVE ZOOLOGY. 



179 



in a very complicated manner, but we have not been able to find any 

 evidence which would Jcad uy to suppose that there is a gradual transi- 

 tion from any one of the rocks into another of the same locality. It 

 seems evident to us that the dioritcs are truly eriiptive rocks, and that 

 their extrusion has taken place since that of the granite and felsite 

 which they penetrate. 



Diabase and molaphyr, so abundant in the neighborhood of Boston, 

 are found in very distinct dikes, cutting all the other rocks, and close 

 the series of eruptive rocks in which there seems to have been, in the 

 order of extrusion, a general progress from silicious to basic rocks. 



The relative ages of the rocks in the region we have explored, as it 

 seems to us, have been pretty clearly established, but the position of 

 the whole series in the geological column is a matter concerning which 

 we have seen very meagre and conflicting evidence. The slates, sup- 

 posed hy some to be primordial, lying between the Charles River and 

 the hills of Medford and Maiden, have not been found, so f.ir as we 

 know, in contact with the eruptive rocks to the northward. On the 

 north side of the Saugus branch of the Eastern Eailroad, between 

 Faulkner's station and Maplewood, the slates arc exposed within about 

 one hundred and fifty feet of the lulls of eruptive rocks upon the north 

 side of Salem Street. The slates dip steeply {66") to the northward, as 

 though they were plunging beneath the other rocks. It may have been 

 this fact which led Prof John W. Webster, many years ago, to assert 

 that the transition rocks (slates, etc.) were overlain hy the porphyries of 

 Maiden. 



^ 



In the rocks of the stratified group, so far as we know, fossils have 

 never been found; and were it not for the fact that the Iloxbury con- 

 glomerate contains numerous pebbles of quartzite, apparently derived, 

 at least in part, from the rocks of tlie stratified grouj), I can sec no good 

 reason for supposing the stratified group to be prc-pahcozf>ic. The same 

 conglomerate contains pebbles of felsitc, but whether any of them are 

 from the felsites now exposed along the northern margin of the Boston 

 basin is a question which, for its satisfactory solution, will require much 

 more thorough and careful work than has yet been done in this region. 



i 



\. 



. 



Conclusions. 



The facts we have observed in the region described in the foregoing 

 paper appear to establish, for that region, the following conclusions : 

 The stratified group contains the oldest rocks of which we have any 



