On Some Dykes Containing Huronite. 47 



and shales of Cambrian age, running almost parallel to their strike. 

 The specimen was taken from near the middle of this dyke, and shows 

 macroscopically a dark greenish grey, rather coarse grained diabase, in 

 which are embedded numerous phenocrysts of altered greenish felspar 

 (Huronite). The crystals of " Huronite," though much smaller than 

 usual, are on the other hand much more abundant, so that it is often 

 difficult to obtain even x small chip of the finer groundmass, in which 

 they are embedded. The felspar of both the larger porphyritic indi- 

 viduals and those present in the groundmass show great alteration, 

 although the polysynthetic twinning lamellae may still be recognized. 

 The decomposition products are mainly sericite and epidote. The 

 specific gravity of these phenocrysts according to Mr. Johnston, i3 

 2.773. The augite when fresh (which is rarely the case except in very 

 minute fragments), is of a reddish colour, and shows distinct 

 pleochroison. A threat deal of chlorite is present. The ilmenite 

 occurs in irregular grains as well as fragments, which have a more or 

 less perfect crystallographic outline and occasional perfect rhombohedra 

 were noticed. The alteration to leucoxene is very characteristic, this 

 resulting form of sphene frequently exhibiting its characteristic brilliant 

 chromatic polarization in thin sections. (1) Besides these larger frag- 

 ments small rounded grains of a brownish grey translucent minera' 

 occur with high index of refraction, and show brilliant interference 

 colours. These occasionally show small granules in the centre of un- 

 altered titanic iron ore, and thus reveal their derivation. Apatite is 

 very abundant, and occurs in colourless prismatic needles which are 

 frequently bent, cracked and broken. Pyrite is also a rather abundant 

 accessory constituent. 



(1) Page 16 A, Report Geological Survey of Canada, 1SS0-2. 



