Oriented rock samples
Matuyama investigated volcanic rocks from Genbudō and other East Asian sites, recording their natural orientations before sampling. Some magnetisations resembled the present field; others pointed nearly in the opposite direction.
References: [1]
The 1929 interpretation
Comparing directions with geological ages and considering later tilting, his 1929 paper inferred that the regional field had been reversed during the earlier Quaternary. It connected remanent magnetisation with a chronological interpretation of magnetic history.
References: [1]
The Matuyama interval
Subsequent work combined rock magnetism with more precise dating to construct a polarity timescale. The most recent long reversed-polarity interval was named Matuyama in recognition of his contribution.