NOVELTY - Glass substrate comprises a down-drawable composition containing (in mass%) silicon dioxide (50-70), aluminum trioxide (5-20), sodium oxide (6-20), potassium oxide (0-10), magnesium oxide (0-10), calcium oxide (above 2-20), and zirconium dioxide (0-4.8), where the ratio of e.g.: silicon dioxide (1 mole) to aluminum trioxide (0.5 mole) is 46.5-59%; and calcium oxide to metal oxide (RO) is greater than 0.3, where RO is a total mass percent of at least one compound including magnesium oxide, calcium oxide, strontium oxide or barium oxide.    USE - The glass substrate is useful for manufacturing chemically strengthened glass substrate, which is useful as a cover glass (all claimed) of a mobile phone, digital camera, digital camera or personal digital assistant, solar cell or fiat panel display, where the cover glass is useful as a touch panel display substrate, window glass, a magnetic disk substrate, a flat panel display substrate, a solid-state imaging sensor cover glass or dinnerware.    ADVANTAGE - The glass substrate: exhibits an improved etching speed and smooth surface, and does not require polishing to be performed after formation, thus can be produced with low cost; and provides the cover glass having improved anti-devitrification properties, mechanical strength and superior quality, with high yield.    DETAILED DESCRIPTION - Glass substrate comprises a down-drawable composition containing (in mass%) silicon dioxide (50-70), aluminum trioxide (5-20), sodium oxide (6-20), potassium oxide (0-10), magnesium oxide (0-10), calcium oxide (above 2-20), and zirconium dioxide (0-4.8), where the ratio of: silicon dioxide (1 mole) to aluminum trioxide (0.5 mole) is 46.5-59%; calcium oxide to metal oxide (RO) is greater than 0.3, where RO is a total mass percent of at least one compound including magnesium oxide, calcium oxide, strontium oxide or barium oxide; sum of strontium oxide and barium oxide is less than 10%; the sum of zirconium dioxide and titanium dioxide to silicon dioxide is 0 to less than 0.07; and boric oxide to metal oxide (R2O) is 0 to less than 0.1, where R2O is a total mass percent of at least one compound including lithium oxide, sodium oxide or potassium oxide. INDEPENDENT CLAIMS are also included for:    (1) a chemically strengthened glass substrate comprising: the glass substrate including a compressive stress layer at its surface, which is formed by a chemically strengthening treatment applied to the glass substrate;    (2) manufacturing the glass substrate, comprising: melting a glass raw material blended so as to have a glass composition (i.e. down-drawable composition) in the frit material; and forming the glass substrate having a plate-shape by a down-draw glass forming method using the melted frit material;    (3) manufacturing the chemically strengthened glass substrate, comprising: preparing the glass substrate; and applying an ion-exchange treatment to the glass substrate;    (4) manufacturing the cover glass, comprising: preparing the glass substrate; and performing etching on the glass substrate; and    (5) the cover glass manufactured by the above method.