THE GOWGANDA LAKE DISTRICT, ONTARIO 659 



all the foregoing series are intruded sills and dikes of diabase, the 

 sill-like form being generally assumed in the sedimentaries. The 

 succession here noted is essentially that found in all the Huronian 

 areas of northern Ontario which have received careful studv. The 

 diabase is of remarkably uniform character in widely separated 

 districts. 



Throughout the Gowganda area, with few local exceptions, the 

 sedimentaries with their sills have been little disturbed. They lie in 

 monoclinal blocks with average dips of about 12 degrees to the east, 



NORMAL SILL DIABASE 



The normal diabase of the sills is, when unaltered, a dark-gray, 

 medium-grained, holocrystalline rock, of which the chief constituents 

 are plagioclase and pyroxene with a very little quartz, micropegmatite, 

 biotite, apatite, and black iron ore. 



The plagioclase, amounting to about 60 per cent of the total, 

 occurs in stout laths with average length of about 0.5 mm,, idio- 

 morphic against pyroxene. Extinction angles place it as medium 

 labradorite, Ab3 5An65. Sometimes zonal growth is shown and in 

 favorable cases the outer zone could be determined as acid labradorite, 

 Ab^jAn^j. The plagioclase is generally somewhat altered, minute 

 scales of white mica being the chief product of alteration. The 

 pyroxene is in irregular grains between the feldspar laths. There 

 appear to be two varieties. Normal augite with the usual high inter- 

 ference colors, high extinction angle, and frequent twinning is the 

 more common. The average size of individuals is about 0.5 mm. 

 In lesser amount occur grains of average diameter 2 . o mm. without 

 twinning, showing nearly always parallel extinction, but in some sec- 

 tions as high as 10 degrees inclination. The maximum interference 

 color is a pale yellow of the first order; the optical character is posi- 

 tive as in the augite ; a faint pleochroism is shown. Bayley describes 

 a mineral in his Pigeon Point rocks^ which seems to be identical. The 

 relation of this pyroxene to the feldspar is the same as that of the 

 augite, but the large size of its grains suggests that it began to crystal- 

 lize sooner. The pyroxene is probably enstatite. The appearance 

 of slightly oblique extinction in rare cases is to be explained in its 

 I Bull. log, U.S.G.S., 36, 45. 



