GEOLOGY OF THE PORT LEYDEN QUADRANGLE II 



Specimen to be made up as follows : quartz 75;^ ; sillimanite 12^ ; 

 and with the long axes of the glistening needles parallel to the 

 rock bands; enstatite 5^; magnetite 3;^, and changing to leucox- 

 ene; pyrite 2^; garnet 2fc, and often completely enveloping the 

 magnetite; together with a little zircon and badly decomposed 

 biotite. Another specimen shows from 85 to 90;^ of badly cracked 

 quartz; i or 2^ each of hornblende, pyrite and sillimanite with 

 a little zircon ; while the rest of the rock is made up of gray, 

 uncrystallized, yellow-stained, decomposition products of horn- 

 blende and biotite. Certain other layers are slightly feldspathic. 

 The most striking features in the composition are the very high 

 quartz content, the almost complete absence of feldspar, and 

 the dearth of dark colored minerals. The rock thus appears 

 to have been an almost pure sandstone which has been meta- 

 morphosed to a quartzite. The rock is highly cataclastic and 

 that the mass has been subjected to a great pressure is proved 

 by the local folds and by the general crushed appearance of the 

 exposures [see pi. i]. In the field the rock has a decidedly sedi- 

 mentary look and we have here a fine example of the Grenville 

 which does not appear to hai^e been very profoundly changed 

 from the original sediment. Where these gneisses grade into 

 the surrounding mixed gneisses some feldspar and a larger per- 

 centage of dark colored minerals are present. 



The Grenville rocks in the Lyonsdale area are chiefly feldspar- 

 garnet-mica gneisses. The gneissic structure is here greatly 

 accentuated by the alternation of light and very dark gray layers 

 which are usually from a few inches to a foot or more in thick- 

 ness. A thin section of the light gneiss shows quartz 75^^; feld- 

 spar 15 to 18;^, mostly oligoclase to labradorite, but with a little 

 microperthite ; biotite 5;^, and garnet 2 or 3;^. In the dark 

 gneisses the quartz is proportionately less prominent while the 

 biotite may run as high as 35 or 40;^. Both the light and dark 

 rocks are often very garnetiferous. In some specimens a few 

 small flakes of graphite were noted. The original sediments 

 here were probably somewhat carbonaceous shales and shaly 

 sandstones. 



The rock bands show a northward dip at a high angle and a 

 strike of n. 40° e. On the north side the exposures are not good 

 but the Grenville does not seem to be sharply separated from 

 the surrounding rocks, while on the south side, along Moose 

 river, there is exhibited a very sharp contact between the Gren- 



