406 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



The unstratified rocks were assigned an igneous origin without excep- 

 tion. They were regarded as having been formed during diverse ages, 



+ 



the author holding that 



'* their intnisiou among the stratified rocks affords an important clue for de- 

 termining their relative ages. It is ohvious, however, that the intrusion of 

 the former among the strata of the latter, only proves that the unstratified rock 

 was formed posterior to the stratified one/' 



While we believe that the above quotation from President Hitch- 

 cock's work is essentially sound in its views, it will be seen in the sequel 

 that considerable work has been done in this vicinity from a diametri- 

 cally opposite point of view. 



President Hitchcock further held that all these rocks were 



*' merely varieties of the same melted mixture, whose peculiarities resulted 

 from the modes in which they were cooled, and crystallized, and intruded 

 among the stratified rocks On this supposition we are no longer sur- 

 prised to find it impossible to draw any definite line between the dilFcrent va- 

 rieties, nor to find them all united in the same mountain mass." 



He inclined to the opinion, that all the unstratified rocks of Massa- 

 chusetts belonged to a single family, but concluded to treat of them all 

 under four divisions, viz. greenstone, porphyry, syenite, and granite. 

 The order of production according to him, beginning with the lowest, is 

 as follows : granite, syenite, porphyry, and greenstone. He further 

 states : — 



'^ Porphyry, however, passes hj insensible gradations into ?ieuite ; but the 

 change commonly takes place in a vertical and not in a horizontal direction." 

 (I. c, pp. 402-404.) 



The greenstone was regarded as a mixture of hornblende and feldspar, 

 although some at Nahant was thought to contain augite. This rock 

 was said to be mixed with syenite in every conceivable mode, and to 

 pass into it and into granite. The former rock was in general the 

 younger, although cases were observed in which the latter held frag- 

 ments of the greenstone. A special case is cited, a little west of Mar- 

 blehead village, and explained as follows : 



*' They certainly appear as if the greenstone had been partially melte<] down 

 in the granite ; thougli the heat was not great enough to complete the fusion. 

 Or rather, may it not be probable, that the perfect fusion of tlie rock out of 

 which these unstratified ones were produced, gave rise to the granite ; while 

 those portions that were not so entirely fused as to admit of entirely uaw and 

 perfect combinations and crystallizations, might have formed those portions of 

 the rock which I call greenstone." 



