§ Amarthon, Dolamarth ’Mount Doom’ (TI:343)
Both forms include ammarth ’fate, doom’ (MBARAT-). According to LotR App.D mb became m in all cases, but still counted as a long consonant for purposes of stress […], and is thus written mm in cases where otherwise the stress might be in doubt. The word amarth may only be stressed on the first syllable, so stress is not in doubt here and it can be safely written like that. Amarthon is stressed on the second syllable, so mm is not important here either. But one wonders why Dolamarth is written this way, since m < mm < mb would attract the stress to the penultimate syllable and one would expect *dolámmarth, not *dólamarth.
In any case, the first form shows the name-formative or augmentative suffix -on (PE17:43), compare Andon above (2.11).
The second form (written above the first) has initially dôl ’head’ (NDOL-) or dol ’head, hill’ (RC:433), ’head; often applied to hills and mountains’ (Silm.index), S. dol (doll) (PE17:36); and amarth in genitival position.