Abstract
Several silica sand deposits are known, in Minami-sakugun, Nagano Prefecture, among which two are now working. These deposits in this district are derived not from quartzose sand or sandstone but from contact metamorphosed Paleozoic chert which was hydrothermally altered later. The deposits are always found as roofs or as xenoliths of Tertiary granitic intrusives. As a result of contact metamorphism, the grain size of quartz of chert becomes as large as recognizable with naked eye.
In this paper, the relation between the general geology and the ditsribution of the deposits, are described and the genesis of the silica sand deposits is also discussed.