Note:  Do not rely on this information. It is very old.

Quartzite

Quartzite, a very hard, compact, granular rock, seldom schistose, but occasionally fossiliferous, which occurs among the older sedimentary rocks, and is, in most cases, obviously an altered sandstone. It differs from the harder sandstones such as sarsen-stone (q.v.) only in showing when under the microscope signs of the partial fusion of some of its constituent quartz-grains. In some cases there is evidence of induration having resulted from the transfusion of heated silicated water rather than from dry heat or pressure.