448 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



is used — and then cautiously ~ fur idcnlilying eqiuvalent beds. Has any 

 one 2;rf;i'c(/ l>y cai'eful oliservation that crystals of staurolite, cyanite, or audalu- 

 site, are restricted to rocks of a certain geological period ? Assumptions and 

 opinions, liowever strongly emphasized, are not proofs. It is no objection to 

 strutigraphical evidence that it is dillicult to obtain ; is very doul*tJ'ul on ac- 

 count of th(j diiUeulties ; may take scores of years in New England to reach 

 any safe conclusions. It nmst be ol*iained, whatever labor aii<l care it costs, 

 l)t;fore the real (n'der and relations of the rocks can be known. Until then, 

 lithology may give us guesses, but nothing more substantial. Mr. Hunt's 

 arguments with reference to the White Mountain Series, us urged by him in 

 1870, will be found in this journal, IT, L, 83. Both there, and in his ad- 

 dress, may be seen the kind of evidence with M^hich he fortifies, or sup- 

 plements, that based on the c])aractor of tlie rocks. Direct stratlgraphical 

 investigation over the region itself, in which all ilexures, faults, and uncon- 

 foi'uiabilities have been thoroughly investigated, is not among the foundations 

 of opinion which he brings forward. He endeavors to set aside the objection 

 to his views suggested by the existence of Devonian or Helderberg rocks in 

 central and northern Kew Enghmd ; but he presents, for this purpose, only 

 some general considerations, of little weight, instead of delijdte facts as to the 

 extent and variety of metaniorphic strata that are part of, because confonnaljle 

 to, these Helderberg beds. Had he studied up these stratlgraphical relations 

 with the care reiiuisite to obtain the truth, an<l all the truth, perhaps he would 

 not longer say — it is ' contrary to my notions of the geological history of the 

 continent to suppose that rocks of Devonian ago could in that region have 

 assumed such litbological characters.' Notions often lead astray." (See also 

 Am. Jour. Sci., 1871*", (3) IL, pp. 205-207; 1872, IV., pp. 104, 105.) 



In replying to Professor Dana, Dr. Hunt states (Amer. Jour. Sci., 



1872, (3) IV., p. 51) : — 



"With regard to New l^.ngland rocks, Prof. Dana asserts that 'there are 

 gneisses, mic^ schists, and chloritic and talcoid schists in the Taconic series.' 

 1 have, however, shown in my address that Emmons, the author of the Taconic 

 system, expressly excluded therefj'om the crystalline rocks, which he included 

 in an older primary system ; excepting, however, certain micaceous and tal- 

 cose beds, which he declared to be recom[)osed rocks, made up from the ruins 

 of the ].rimarv schists, and distinguished frtnu tliese by the absence of tlte 

 charncteristic crvstalline minerals which belong to the Green Mountain pri- 

 mary schists. Again, Prof. Dana states that I make the crystalline schists 

 of the White Mountains a newer series than the (Jreen Mountain rocks. A 

 cai-eful perusal of my address will show that I nowhere assert that the rocks of 

 the third series, on my line of section, are younger than the seccmd series. 

 Such a view of thei]-" relations has, however, been maintained for the last 

 generati(m by the Messrs. Pogers, Logan, and many others, all of whom 

 assigned the'crystalline schists of the White Mountains to a higla^r geological 

 horFzon than the Green Mountains My ' chronological arrangement ' of 



