PETEOGRAPHY OF ONTARIO. 49 



their polysynthetic structure, which is well brought out by polarised light. The twin 

 lamellte are often developed iu two directions nearly perpendicular to one another, form- 

 ing a cross-barred structure quite unlike that of microcline. The general habit, inclusions 

 aud decomposition products of the plagioclases are very much like those of the potassium 

 felspar, and need no special notice. No attempt has been made in this investigation to 

 divide up the plagioclases of the acid rocks into subspecies. 



Biotite. — Of the basic silicates biotite is the commonest, though closely followed by 

 hornblende. This mica is usually brown, though sometimes green, and in sections not 

 parallel to the chief basal cleavage is known by its enormous absorption when its cleavage 

 is parallel to the section of the lower nicol, the upper uicol having been removed. The 

 optical axes, which are seen in basal sections, usually include so small an angle as to give 

 the idea of a hexagonal mineral, though sections of a hexagonal outline are rare in the 

 granites. Except small brown or black scales wedged in between the cleavage lamellae, 

 inclusions are uncommon. Biotite appears to weather to a confused fibrous substance of 

 greenish or brownish color, with oxides of iron. In one case fragments appear to be 

 included iu epidote, as though remnauts of metamorphosed crystals. 



Muscovite is much rarer than biotite, aud from its colorlessuess or faint color, and 

 hence want of dichroism, is less obvious under the microscope. A peculiar moiré lustre in 

 polarised light is characteristic' In habit it is much like biotite, with which it is fre- 

 quently found intergrowu. In some instances the muscovite seems to have originated 

 by the decomposition of potassium felspar, as shewn by its position within orthoclase 

 crystals. 



Hornblende. — This often accompanies or replaces biotite, and is readily known by its 

 cleavage and optical properties. It is usually green, but sometimes brown, with strong 

 pleochroism. It often weathers to dirty greenish or brownish chloritic substances, with 

 the separation of magnetite or other oxides of iron. In one specimen it has changed to 

 pale greenish epidote, which forms a perfect pseudomorph, with the exception of an un- 

 changed remnant of hornblende in the centre. In one section small fibres of glaucophane 

 occur, haA' ing intense pleochroism (indigo blue, drab, colorless). 



Accessory Minerals are numerous, such as magnetite, which occurs with rectangular 

 cross-sections or in irregular grains. In some cases the magnetite is probably of primary 

 origin ; in others it is formed, often with brown or red oxides of iron as well, by the de- 

 composition of hornblende or other minerals rich in iron. It is occasionally titaniferous, 

 as shewn by an aureole of greyish leucoxene. Titanite also occurs in brown slightly 

 dichroic cross-sections of the usual sharpened oval shape. Apatite is very common, but 

 in minute amounts. Its slender needles must have been the earliest crystallisations, 

 since they transfix the other minerals impartially. Pyrite and other metallic su.lphides 

 are occasionally found in disseminated grains or crystals. As products of decomposition 

 we find secondary quartz, epidote, various serpentinous and chloritic minerals, oxides of 

 iron and (rarely) calcite. 



The varieties of granite so lucidly described by Roseubusch" are found in our drift : — 



Biotite-hornblende-granites are commonest. 



Hornblende-granites, mica being replaced by hornblende, come next in frequency. 



' Von Lasaulx, Eiaf. in die Gesteinslehre, p. 52. - Mikr. Phys., pp. 18, etc. 



Sec. iii, 1887. 7. 



