HOLYOKEITE, A PURELY FELDSPATHIC DIABASE 509 



of two sizes, both elongate blades with ragged ends and irregu- 

 lar sides; the finer about o.03 mm long; the latter having some- 

 times quite regular crystalline outlines. Scattered in this 

 network are distant feathery groups of plagioclase crystals of 

 first consolidation which are just visible to the eye. One of 

 these larger crystals cut parallel to M (010) showed the optical 

 figure of albite. 



All these feldspars are lightly dusted with blades and grains 

 of a secondary mineral of low polarization, probably zoisite. 

 And there is a very little granular limonite scattered through the 

 rock, but no accumulation of it, or appearance of any chloritic 

 mineral to indicate the former presence of magnetite and augite 

 or hornblende. There are no brightly polarizing blades that 

 could be referred to a colorless bisilicate. 



As in the peculiar red diabase from Cheapside, near Green- 

 field, described in the monograph 1 above mentioned, the small 

 round steam cavities are lined by a secondary growth of albite in 

 fresh limpid crystals. 



There are other fragments associated with the Holyokeite 

 which are black, aphanitic, and show under the microscope a simi- 

 lar texture, but contain large spots of a green chloritic mineral. 



The chemical composition of the rock is shown by the fol- 

 lowing analysis I, by Mr. Hillebrand. 



If we follow the calculation of the analyst and assign the 

 sulphur to 0.40 chalcopyrite and 0.06 per cent, pyrite, and then 

 calculate the phosphoric acid as apatite, the titanic acid as 

 ilmenite, the potash as orthoclase, and assign the carbonic acid 

 to the magnesia and most of the calcium, we shall account for 

 30 per cent, of the analysis. 



It is interesting that the remaining 70 per cent, has the com- 

 position of an albite of exceptional purity, and the only feldspar 

 determined in the slide was albite. 



Si0 2 - - - 67.89 

 A1 2 3 - - 20.87 

 Na,0 - - - 11.24 



100.00 

 1 Loc cit., p. 431. 



