Fossil diatoms found in two diatomite samples taken from two vertically located strata about one meter above the surface of the road on the top of Katakura-toge (Katakura Pass), Mochizuki Town, Nagano Prefecture were investigated.
In these saamples,49 taxa in c luding5 unidentifiedt axa were recorded. They belong to the 18genera and 7 families. Among those taxa, two species, Cocconeis diminuta and Melosira distans are noticeable. The former alkalibiontic form is abundant,40.3 % in the upper stratum but much less in the lower stratum,6.7 %. On the other hand, the latter acidophilous form is abundant,50.8 % in the lower stratum, but less 23.6 % in the upper stratum.
Based mainly on those facts, the past sedimentary environ m ent of those strata is assumed to be a lake or marsh in which the transition occurred from acidic water caused presumably by volcanic activities to alkaline water caused by successive plant growth.