From the Bureau of Mines report by J. J. Forbes, G. W. Grove, J. M. Ankeny, and H. B. Lindeman
An explosion that traversed a limited area in Nos. 16, 17, and 18 right entries off north drip caused the death of 63 of the 350 men in the mine at about 10:40 Monday morning. Probably all died from afterdamp although eight were also burned. Eighteen men escaped from the area without help, as did 12 others from adjoining workings. The mine had been idle for two days, but the fireboss reported no gas in the affected headings. The first word was a telephone call to the superintendent from underground that the air was reversed, and the foreman was sent to investigate. He met a group of men coming from the affected section and took some of them with him to check the ventilation. They found the airlock doors blown inby on 17 left, causing a direct shorting of the air. They replaced one door and went on making repairs to other doors until stopped by smoke and dust in 18 right.
Help summoned by the superintendent arrived, and apparatus crews made explorations. Air was brought into the area by putting up canvas stopping and a smoldering fire in a jacket was extinguished. Footprints in the dust led to discovery of a canvas barricade in 16 right by an apparatus crew and then a second stopping partly completed 52 feet inside. Thirty-four bodies were found here about 10:30 p.m. Some of the men had been alive as long as seven hours after the explosion, but all had died from afterdamp that had come in through other openings not sealed off. All bodies were found and removed with 22 hours.
A body of methane was released from overlying rock strata by a fall in a room. Pillars were not removed in this mine, and large areas of open working were poorly ventilated. The gas mixture reached a haulageway and was ignited by an arc from a trolley locomotive. The explosion was not strong enough to raise enough coal dust into the air to propagate the flame beyond the gassy area although no rockdusting had been done. The mine was moderately gassy, and it was the practice to fence off gassy places until ventilation could be arranged to clear them.
Additional Resources:
http://patheoldminer.rootsweb.ancestry.com/camsonman1.html
geocities.com/lydick_1999/Sonman/Sonman.html
en.wikipedia.org/wiki/Sonman_Mine_explosion
gendisasters.com/pennsylvania/5578/portage-pa-mine-explosion-kills-63-workers-july-1940
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