Page:A Treatise on Geology, volume 2.djvu/106

 92 varieties are classed according to the predominance of magnesia, alumina, potash, or oxide of iron, will show.

Granite, of the ordinary kind, compounded of quartz, felspar, and mica, varies greatly in the proportion of these substances, yet the fused glasses from which these various products have crystallised, might differ only by small variations in the proportions of the ingredients.

Granite, composed of quartz 2 parts, felspar 2 parts, and mica 1 part, would, according to Mr. de la Beche's calculation, be represented in column 1. of the table below; and porphyritic granite, composed of quartz 2 parts, felspar 3 parts, and mica 1 part, in column 2.; and we have added binary granite (felspar 3 parts, and quartz 2 parts) in column 3.