No. 6. J HARRINGTON NOTES ON DYKES. 319 



III. River St. Simon. (Plate, fig. 2.) This specimen is 

 from a fine-grained, greyish-black dyke which cuts the Green 

 Lake band of crystalline limestone on the St. Simon, a small 

 tributary of the North River, in Terrebonne County east of 

 Grenville. I am indebted for it to the Director of the Geologi- 

 cal Survey. The dyke probably belongs to the same set as the 

 Grenville ones just described, its general structure being the 

 same, but it has apparently undergone very little alteration, the 

 section being beautifully clear and transparent. With the micro- 

 scope it is seen to consist of a network of plagioclase feldspar, 

 with augite, magnetite and apatite (?) and a very little viridite. 

 The feldspar as seen in the section is perfectly transparent and 

 colourless, and with the polariscope shows a beautifully banded 

 structure. In places it contains microlites which are possibly 

 apatite, and also a few vapour- or gas-cavities, generally in groups. 

 The augite is pale greyish-brown frequently penetrated by 

 blades of feldspar and often containing groups of minute grains 

 of magnetite. It appears to constitute about half the rock. 

 The magnetite occurs mostly in irregular grains and masses of 

 most fantastic shape, but now and then in rude crystals and rod- 

 like forms. In some cases it is seen to be penetrated by blades 

 of feldspar. (See figures on next page.) 



The viridite is not very abundant and looks as if derived from 



Under a lens the rock is seen to consist of a greenish-white feldspar 

 with a scaly fracture, mingled with grains of pyroxene, occasional 

 plates of mica, and grains of pyrites. It contains no carbonates. 

 Two analyses of portions of the dolerite from dykes differing a little 

 in texture gave as lollows : 



Silica 50.35 50.25 



Alumina 17.35 1 3210 



Peroxyd of iron 12.50 / 



Lime ."" 10.19 9.63 



Magnesia 4.93 5.04 



Potash 69 .58 



Soda 2.28 2.12 



Volatile 75 1.00 



99.04 100.72 



« The iron in these analyses, although given above as peroxyd, 

 exists in the form of protoxyd, and in the second specimen, in part as 

 a sulphuret." (Am. Jour, of Sci., 1864, 2nd Ser., Vol. xxxviii, p. 174.) 

 Which of the analyses applies to the specimen from Range IV and 

 which to that from Range V is not stated. 



