418 Scientific Geology. 



modes : first, as protruding irregular masses ; secondly, as overlying 

 masses; and thirdly, as veins. The first and last modes are most 

 common. 



Since most of the greenstone in the eastern part of the state is not 

 connected with stratified rocks, it must be referred to the first of these 

 modes ; except in those comparatively few instances, where it forms 

 veins in the other unstratified rocks. I have never been able to find 

 a satisfactory example, in which the greenstone distinctly overlies 

 either porphyry, sienite, or granite;* although in numerous instan- 

 ces I have found a gradual passage from this rock into the two latter : 

 But this is as likely to take place laterally as in a vertical direction. 

 Examples of this gradual transition between these rocks are common 

 south of the Blue Hills, as in Randolph, Stoughton, &c. 



Wherever I have seen this rock associated with the graywacke and 

 argillaceous slate in the eastern part of the state, it either occupies 

 veins, or protrudes itself in some other form, among, or between, the 

 strata. Professor Webster, however, says, that it is sometimes su- 

 perincumbent upon the clay slate in Charlestown.f It has there also 

 the appearance of being regularly interstatified with the slate : But I 

 am satisfied that this is a deception ; that is to say, these supposed beds 

 are connected with some unstratified masses. Yet I think it ex- 

 tremely probable that some of the greenstone in the vicinity of Boston 

 has resulted from the fusion of clay slate ; and perhaps it is possible 

 that a particular portion of the slate might be converted into green- 

 stone, while that around it might remain but little changed ; and in 

 such a case, the altered rock might at the surface appear interstratifi- 

 ed with the other. In such a case, however, we should rather expect 

 that the slate would be converted into hornblende slate : and Professor 

 Webster says that some of the clay slate in Charlestown does pass 

 into hornblende slate : and I would remark that much of the green- 

 stone in the vicinity of Boston resembles exceedingly that variety 

 which is associated with hornblende slate: indeed, as already stated, 

 some of it is associated with hornblende slate. But for the most part it 

 appears to have been subject to so entire a fusion that the schistose and 

 stratified structure is lost ; and hence it seems most proper to describe 

 it as unstratified greenstone : although if it be true that all green- 



* Professor Webster says that it overlies compact feldspar in Charlestown. Bos- 

 ton Journal of Philosophy, vol. 1. p. 282. 



tBost. Journal Philosophy, vol. 1. p. 285. 



