In one example from January 15, 1925, R.A. Finch (then the head of the Lassen Volcano Observatory) wrote:

"There have been several reports recently concerning activity of Glass Mountain...about 75 miles north of Lassen Peak, in the Modoc Lava Beds. Rather new lava flows are to be found nearly all the way between Glass Mountain and Lassen Peak. Forest Service officials report about half an acre of land covered with pumice that is very hot. By digging but a little way into the pumice, much higher temperatures can be reached... [Rancher George W. Courtwright] reports that numerous earthquakes occurred in January and February 1910, and that during the shaking flames were observed over the mountain. In the spring of the same year he reports finding "blue mud" on vegetation on the mountain slope."

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All of these anecdotes and observations might rightfully lead a scientist to suspect that Glass Mountain (pictured here) had been an active volcanic site within recent times. Nowadays, thanks to detailed mapping, geochemical and petrologic studies, and various dating methods, we know that this eruption occurred about 950 years ago. Tephra produced by explosions early in the eruption deposited pumice and ash several meters thick near the vents and as much as several millimeters up to 40 km (25 mi) to the northeast. While local tribes almost certainly had a chance to observe that eruption, the rancher's 20th-century observations require other explanations. For example, tephra deposits are capable of retaining heat for some time after eruptions, and earthquakes are common in volcanic areas even if they don't lead to surface activity.

To read more early observations of California's Volcanoes, check out the Lassen Volcano Observatory reports in the 1926-1935 editions of The Volcano Letter: https://pubs.usgs.gov/unnumbered/70246900/report.pdf