COETLANDITE. 



347 



light color, strong- prismatic cleavage and abundant twinning. The broad 

 hornblende surfaces are also replaced by an aggregate of the pp-oxene 

 gi-ains, and thus passes into the second type of the series. The normal 

 decomposition is into talc, and broad bands centrally blackened l)y mao-- 

 netite grains pass through the olivine and pyroxene alike. 



The relation of the luster-mottled hornblende to the blotite is less clear. 

 It seems at first sight to be a superficial change, and the mica, now green 

 and now brown, is developed In the cleavage planes of the hornblende. 

 But as it is luster-mottled by all the other minerals with fresh borders, it 

 was quite certainly formed originally by some change in conditions of 

 cooling, and simultaneously with the hornblende. Perhaps it is to be looked 

 at as a contact-metamorphic eff"ect, brought about by the introduction of 

 fluorine from without. There is associated witli the normal rock a massive, 

 frial)le, granular rock, made up of fibrous hornblende and bright emerald- 

 green pyroxene, like that found on the western border of the hornblende- 

 granite. Enstatite occurs but rarely. Calcite is quite common, as soon as 

 change sets in. Feldspar is wholly wanting. 



An analysis of the finer-grained and fresher portion of the mass was 

 made by Mr. L. G. Eakins : 



Analysis of cortlandite from Belchertown. 



