The rock Lherzolite has been described by Prof. Zirkel in his valuable Beiträge zur Geologischen Kentniss der Pyrenäen (Zeitschrift der Deutsch. Geol. Gesel., vol. xix. p. 68), but is generally passed over with the briefest mention or entirely omitted in English works on Geology. Even in Cotta's “Rocks Classified and Described” it is barely noticed, and the word is left out in the index. On this account, and seeing that, so far as I am aware, no description of its microscopic structure has yet been published, a notice, embodying the results of Prof. Zirkel's paper, and of a brief visit of my own to this not very accessible locality, may be useful to students.