THE IGNEOUS KOCKS. 



495 



The composition and structure of even the more altered forms are proof that 

 all have originated by ordinary processes from an igneous magma. 



In order to determine the chemical diti'erences that exist between the 

 augitic phases of the greenstone and those phases in which all the augite 

 has been changed to amphibole, analyses of two specimens were made by 

 George Steiger in the Survey laboratory. His results are as follows: 



Analysm of gnriiatoiirs. 



I._No. 22168. From SW. i sec. 12, T. 47 N., R. 27 W. Massive greenstone contain ing pile .augite 

 in fairly large quantity. 



II.— No. 22159. Ficm a point near No. 22168. Typical uonaugitic greenstone, like the greater 

 portion of the mineis' "dinrites." 



Evidently there is no great difference in the composition of these two 

 rocks. The more altered one contains less alumina and lime than the less 

 altered one, and a little more iron oxide and magnesia; otherwise the two 

 are similar. These analyses afford no evidence confirmatory of the view 

 that in the formation of amphibolites from diabases iron oxides are removed 

 and condensed into ore bodies.' 



The rocks of the other eastern knobs present {e\v features different 

 from those described above. From six different knobs specimens were col- 

 lected whose slides show remnants of nearly colorless augite when examined 



'Report of State Board of Geological Survey for 1891 and 1892, Lansing [Mich.], 1893, p. 180. 



