On Some Dykes Containing Huronite. 39 



to this parallel arrangement of the fibres, the serpentinous substance 

 gives a faint but definite reaction with polarized light. The meshes of 

 the net-like structure thus produced are filled with more finely devel- 

 oped scales and fibres of serpentine which are nearly, if not, quite 

 isotropic. These decomposed grains are often seen embedded in the 

 fresh augite. The ilm*mite occurs in large irregular fragments or in 

 small more or less rounded granules and in both cases shows character- 

 istic alteration to leucoxene. The leucoxene is of the usual opaque 

 grey colour, but sometimes brownish grey, and frequently show, 

 especially in the thinner portions of the slide as also the smaller frag- 

 ments, the brilliant chromatic polarization of sphene of which it is 

 simply a variety. 



5. Locality. ^ mite north of Murphy Lake, Timber Limit, 90, 

 District of Algoma, Ont. 



The specimen is from a dyke cutting rocks of Huronian age. The 

 matrix is a normal datk green diabase whose ophitic structure is mega- 

 scopically apparent. A freshly exposed surface shows the Huronite to 

 be of the usual pale yellowish green colour, while the less altered por- 

 tions of the crystals have a more or less pinkish or flesh red colour. 

 In many of these individuals a somewhat indistinct cleavage and a 

 rather faint striation due to multiple twinning may be seen. The 

 matrix weathers a brownish colour while the phenocrysts become a 

 dull opaque greyish white thus rendering portions of this rock which 

 have been subjected to atmospheric action very conspicuous. 



Microscopically, the Huronite is seen to be labradorite which has 

 undergone more or less " saussuritization." A narrow border usually 

 surrounds these phenocrysts of labradorite which is free from the pro- 

 ducts of decomposition, but immediately within this rim is a zone or 

 band where the alteration has been extreme and here the resulting 

 zoisite, epidote and sericite replace nearly, if not quite, all of the origi- 

 nal felspar. The epidote and zoisite are present in irregular grains or 

 masses, while the sericite, as usual, occurs in scales and plates. All of 

 these alteration products have a more or less definite arrangement. The 

 grains and imperfect crystals of epidote and zoisite are usually elongated 

 in a direction corresponding more or less with the twinning striations 



