154 Coleman — Nej^heline and other Syenites in Ontario. 



a weathered olivine crystal is modified so that the parts near- 

 est the olivine are more strongly dichroic than the rest, bright 

 green and orange-brown in the two directions. 



The fine-grained dikes of brownish plagioclase rock are not 

 very sharply defined as a rule, perhaps because they were 

 erupted before the mass as a whole had completely cooled 

 down. In many cases these rocks are specked with white or 

 pale flesh-colored spots as if amygdaloidal, and they often con- 

 tain what seem to be fragments of an older, tine-grained reddish 

 rock. 



The general mass of these rocks consists of some greatly 

 weathered, lath-shaped plagioclase, partly with a radiating 

 arrangement, enclosing biotite, augite, and magnetite in larger 

 amounts. In this ground mass are often well-formed crystals 

 of augite, sometimes in groups, pale green or brown, somewhat 

 dichroic and with a zonal structure ; and of dark brown horn- 

 blende. The lighter patches, suggesting amygdaloids, are com- 

 posed chiefly of plagioclase, often with well-shaped prisms 

 projecting inwards, the center being of some transparent 

 mineral having low double refraction, perhaps a zeolite. 

 There are a few prisms with parallel extinction, probablj^ 

 nepheline, though so badly weathered as to leave their char- 

 acter uncertain. Without an analysis it would be difficult to 

 place this rock with certainty, so for the present it will be left 

 unnamed. 



The dikes of dark gray or black diabase and diabase porphy- 

 rite have been little studied. The only one of which thin 

 sections have been made is an olivine diabase with compara- 

 tively little augite, often in slender fibers or prisms having the 

 usual extinction angle, but sometimes as broader portions 

 between feldspar laths. The magnetite, too, has elongated rod- 

 like forms, and when the numerous needles of apatite are 

 included, there is evident a tendency to elongation in almost 

 all the constituents of the rock. The large well-formed crys- 

 tals of olivine, still fairly fresh, are, however, an exception to 

 the rule just mentioned. These quite fresh rocks are probably 

 of Keweenawan age like most of the diabase dikes on the north 

 shore of Lake Superior, while the other eruptives described 

 appear to be older, though not so old as the Huronian schists 

 which they penetrate. 



It is believed that with the possible exception of the gabbros, 

 which ma}^ be older than the syenites, and the diabases which 

 are distinctly younger, all the rocks which have been referred 

 to belong to the same magma and represent phases of differ- 

 entiation. The dikes of heronite or analcite tinguaite, though 

 found several miles to the east of the nearest syenite, are to be 

 looked on, no doubt, as split off from the large mass described. 



