A.ECHEAN IliT FELOH MOUITTAIN DISTRICT. 



397 



The following complete analysis shows the chemical character of a rep- 

 resentative specimen of amphibolite : 



Analysis of amphibolite. 



[By Dr. H. N. Stokes, U. S. Geol. Survey.] 



' Ba, Sr, Li, CI, S, SO3 were not looked for. 

 No. 1. Specimen 36407, Lalce Superior Division, U. S. Geol. Survey, 1140 N., 1000 W., sec. 32, 

 T. 42 N., E. 28 W., Upper Peninsula of Michigan. 



From this analysis it appears that the rock has essentially the compo- 

 sition of diabase or basalt. The composition of the amphibolites, as shown 

 by the above analysis, and their field relations leave little room for doubt 

 that they are old dikes of basic rock. 



Their present crystallization is of course not that due to original 

 cooling, since among other reasons it bears no relation either to their 

 thickness or to distance from the walls. The evidence of complete recrys- 

 tallization in jjlace after consolidation which they thus aiford, and the 

 unquestionable community of origin between their foliation and that of the 

 gneisses, are significant facts in the metamorphic history of the Archean of 

 this district. 



It is for this reason that they are described with the Archean and not 

 with the intrusives. Whether they are really Archean intrusions and not of 

 Algonkian age can not, perhaps, be known with certainty. Basic rocks 

 having approximately the same composition are known to have penetrated 

 the Algonkian, but they have not undergone the same recrystallization. 

 These last besides have their known analogues, equally unmetamorphic in 

 the Archean itself For these reasons it seems probable that the amphib- 

 olites were intruded into the Archean before the Algonkian rocks of this 

 district were deposited. 



