Abstract
The mylonitic character of the peridotites of St. Paul's Rocks on the Mid-Atlantic Ridge was reported in a contribution by the writer (Tilley, 1947), these ultrabasics then being referred to as dunite or peridotite mylonites. The available chemical analytical data were assembled in that paper and a new analysis provided of a two-pyroxene representative. The type carrying porphyroclastic amphibole was represented by an analysis by Washington (1930) but it was shown that this analysis erred in its alkali determination, and that the reported occurrence of jadeite could not be confirmed.