A verb translated “to-gush” or “flow” appearing in the Lament of Akallabêth (SD/247 SD/311), in later writings attested only in the form phursâ. This form superficially seems to be the aorist of the triconsonantal verb phurus-. Its two draft forms phurusam > phurrusim further support the idea that the verb stem is phurus-. However, ordinarily aorist forms end in a short -a, so perhaps there is something unusual about this formation. It also lacks the usual plural-verb suffix -m appearing in first-draft version of the Lament, which would ordinally be required to agree with the plural subject. Since phursâ appears with the subjunctive or optative prefix du-, perhaps it is a special (infinitive?) form of the verb.
Andreas Moehn instead suggested (EotAL/PHUR) the verb stem may be phursâ- instead of phurus, so that it is a derived-verb. This isn’t consistent with earlier forms, but it is possible that Tolkien changed his mind about the basic verb form.