10-Porphyritic granite

Granite rock with two grain sizes, containing large crystals (phenocrysts) of orthoclase feldspar (reddish) and smaller granite-size grains of quartz (clear) and orthoclase (pink). Rock crystallized in two stages, with slow cooling and more rapid cooling producing coarse and fine crystals, respectively. Rock crystallized at great depth in Canada, 1 to 3 billion years ago. (Compare with no. 7.)