EASTERN MASSACHUSETTS. 407 



It appears from his I'emarks in connection with this, that in his opin- 

 ion a more or less perfect fusion of the same materials may have been 

 the principal cause of the production of greenstone, syenite, porphyry, 

 and granite from them. 



The unstratified rocks were said to " occur in three modes : first, 

 as protruding irregular masses ; secondly, as overlying masses ; and 

 tliirdly, as veins." The greenstone he held to be principally of the first 

 class, but considered that it also occurred in veins. In regard to these 

 points he thus expresses himself : — 



"Wherever I have seen this rock associated with the graywacke and argil- 

 laceous slate in the eastern part of the State, it either occupies veins, or pro- 

 trudes itself in some other form, among, or between, the strata It has 



there also the appearance of being regularly interstratified with the slate. But 

 I am satisfied that this is a deception ; that is to say, these supposed beds are con- 

 nected with some unstratified masses. Yet I think it extremely 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 greenstone, while that around it might remain but 

 little changed ; and in such a case, the altered rock might at the surface appear 

 interstratified with the other." 



On Nahant the presence of two sets of trap dikes was noticed, and it 

 was held that the slate was in the form of a clay at the time of the in- 

 trusion of the greenstone which altered it. The apparent distinct strati- 

 fication (jointing'?) of some greenstone on this promontory was regarded 

 as the result of a concretionary structure. Part of the veins (dikes) 

 were said to run parallel with the strata, and it was thought that they 

 would be regarded by some geologists as being regularly interstratified 

 with the slate. President Hitchcock gives the following as the reasons 

 why he holds the greenstone to be of igneous origin : — 



1. "The resemblance in external characters between some varieties of our 

 greenstone and the products of existing volcanoes." 



2. " The columnar structm-e of greenstone." 



3. " The irregular mamier m which greenstone is intruded among stratified 

 rocks." 



4. " The Mechanical effects of Greenstone upon the Stratified Rocks." 



5. " The Chemical effects of Greenstone upon the Stratified Rocks." 



These views were illustrated by examples. (I. c, pp. 404—442.) 

 The porphyry is divided into four classes : 1. Compact Feldspar ; 

 2. Antique Porphyry ; 3. " Porphyry with a base of compact Feldspar 

 and two or more minerals embedded " ; 4. Brecciated Porphyry. The 

 last he describes as being 



