For the commoner grades of dark-coloured bottles the glass mixture is cheapened by substituting common salt for part of the sulphate of soda, and by the addition of felspar, granite, granulite, furnace slag and other substances fusible at a high temperature.
Glass is made from granulite found in the Meldon Valley, 3 m.
Three divisions are distinguished by some authors - the grey gneiss, the red iron gneiss and the granulite.
The granulite, also called eurite and halleflinta, is the most important of the Pre-Cambrian formation, as it contains all the metalliferous deposits of Sweden.
The metalliferous deposits have generally the form of beds or layers between the strata of granulite and limestone.