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Percent ductility is a measure of the amount of strain that a rock undergoes before losing strength. Ductility varies with lithology. The strongest and most brittle of the rocks is a quartzite or silica cemented sandstone. In contrast, halite is very weak and will undergo large amounts of ductile flow without brittle failure. Figure shows in a generalized manner that the rocks at great depths deform in a ductile manner in contrast to those at shallow depths when they are likely to rupture under brittle manner, within the elastic range. A variety of rocks and their relative ductilities as a function of depth of burial has been compiled by rock mechanists and engineers. Starting with the most brittle there is silica cemented sandstone, dolomite, calcite-cemented sandstone, shale, limestone, and and finally halite.