420 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



year later Dr. Hunt denied that he had held that the White Mountain 

 yeries was younger than the Green Mountain or Iluronian series. (Am. 

 Jour. Sci., 1872, (3) IV. p, 51.) 



In his address before the American Association, Dr. Hunt still held 

 that the gneisses and crystalline limestones of Chelmsford, etc. were 

 Laurentian, and said : 



a rr 



Conglomerates. 



The imcrystalline argillites and sandstones, holding Paradoxides, at Brain- 

 tree, .... overlie unconformably crystalline schists of the second series," 



(Huronian or Green Mountain). (Proc. Am. Assoc. Adv. Sci., 1871 Xx' 

 pp. 10, 32.) ' 



In 1875 Mr. W. W. Dodge classified the rocks of Eastern Massachu- 

 setts as follows (Proc, Bost. Soc. Nat. Hist., 1875, XVII, pp 388- 

 419):-^ 



I. Crystallines. r A ^1 f 

 11. Stratified Eocks ] p' 



Of the crystallines he says : — 



" They underlie unconformably strata holding Paradoxides, etc., and proba- 

 bly formed hill or island ranges yhyj early in the history of this continent. 

 .... For the most part, metamorphism has been so complete that these 

 rocks have lost almost entirely their probable ori^dnal character." 



Of the syenite he states : 



" The abundance of quartz seems to point rather to metamorphic than Igneous 

 condition, and there is probably no doubt that they are chielly of sedimentary 



o^giii Above Spy Pond in Arlington, there are slaty rocks which pass 



through fine grits to coarse sienite by various stages. So, too, in Medford." 



Under the term "porphyry" he makes four divisions, and thinks that 

 some of the slates may have been so altered as to closely resemble por- 

 phyry. He appears to hold that all the rocks included under the term 

 "crystallines" are of sedimentary origin, and have acquired their pres- 

 ent character by metamorphism. The eruptive rocks of this region Mr. 

 Dodge holds to have been derived from the " crystallines," remarking : 



" As a matter of fact, so close is the resemblance in chemical composition, 

 appearance and minerals developed in them, of the eruptives among the slates 

 and conglomerates, to the more fusible portions of the crystallines, that it seems 

 almost unreasonable to doubt that the former were derived from among deep 

 lying hiasses of the latter." 



Of the Brighton melaphyr (amygdaloid) he states : 



^'Whether these rocks as a whole arc of the crystalline stratified class, or 

 whether they have undergone such change as to entitle them more appropri- 



