EASTEKN MASSACHUSETTS. 



407 



It appears from liis remarks in connection with tliis, that in his opm- 

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



the principal cause of the production of greenstone, syenitOj porpljyry, 

 and granite from them. 



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

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

 thirdly, 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 gray wacke and argil- 

 laceous slate iu the eastern part of the Stale, it either occupies veins, or pro- 

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



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

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

 nected with some unstratifted 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 j 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 alterpfl rock might at the surface appear 

 iuterstratified with the other." 



On Nahant the presence of two sets of trap dikes >vas 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 1) 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 tliought that they 

 would be regarded by some geologists as being regularly iuterstratified 

 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 colunmar structure of greenstone." 



3. " The irregular manner in which greenstone is intruded among stratified 

 T-ocks." 



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



5. ** The Chemical effects of Greenstone upon the Stratified Eocks." 



These views were illustrated by examples. (/. 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. Brccciated Porphyry, The 

 last he describes as beiner 



