MINERALS AND GEOLOGY OF CANADA. 5 
spots, bands, &c., as in the varieties of quartz, called agate, blood- 
stone, jasper, and so forth. In Labradorite, or Labrador feldspar, a 
beautiful play or change of colour is observed in certain directions. 
The finer varieties of Opal also exhibit a beautiful and well-known 
iridescence. 
3. Streak.—Under this technical term is comprised the colour of 
the powder produced by drawing or “streaking”? the mineral under 
observation, across a file or piece of unglazed porcelain. The charac- 
ter is a valuable one on account of its uniformity; as, no matter how 
yaried the colour of a mineral may be in different specimens, the 
streak will remain of one and the same colour throughout. Thus, 
blue, green, yellow, red, violet, and other specimens of fluor spar, 
quartz, &c., exhibit equally a white or “uncoloured” streak. The 
streak is sometimes “unchanged,” or of the same tint as the external 
colour of the mineral; but far more frequently it presents a different 
colour. Thus, Cinnabar, the ore of mercury, has a red colour and 
red streak ; realgar, or sulphide of arsenic, has a red colour and 
orange-yellow streak ; copper pyrites, a brass-yellow colour, and 
greenish-black streak ; and so forth. In certain malleable and sectile 
minerals, whilst the colour remains unchanged in the streak, the 
lustre is increased. The streak is then said to be “shining.” Finally, 
it should be remarked, that in trying the streak of very hard minerals, 
we must crush a small fragment to powder, in place of using the file ; 
because otherwise, a greyish-black streak, arising from the abrasion of 
the file, might very possibly be obtained, and so conduce to error. 
It may be observed, however, that all minerals of a non-metallic as- 
pect, and sufficient hardness to resist the file, have a white streak. 
4. Form.—The forms presented by minerals, may be either regular 
or wreguiar. Regular forms are called crystals, whether the minerals 
which present them be transparent or opaque. The term “crystal” 
was first applied to transparent vitreous specimens of quartz or rock- 
crystal; but, as it was subsequently found that opaque specimens of 
quartz presented exactly the same forms, and that opaque as well as 
transparent forms of other minerals existed, the term gradually lost 
its original signification, and came to be applied to all regular forms 
of minerals, whether transparent, translucent, or opaque. Minerals 
of a metallic lustre are always opaque; and many of these, as iron 
pyrites and galena, occur frequently in very regular and symmetrical 
