F. P. Paul described in 1906 (T.M.P.M., vol. 25, p. 309) a nepheline-eudialite-basalt from Shannon Tier, Tasmania, containing an unknown mineral referred by him to Ca2SiO4. Subsequently Bowen (Amer. J. Sci., 1922, p. 30) pointed out the close correspondence between the optical properties of Paul's mineral and of the artificial βCa2SiO4 prepared by Day, Shepherd and Wright. Still later (Geol. Mag., 1927, p. 43) the writer of this note ventured to apply the name shannonite to the mineral of this singular occurrence. Since that time through the kindness of Mr. P. B. Nye, Government Geologist of Tasmania, a specimen of the rock described by Paul from Shannon Tier has been obtained and examined. That the specimen examined is identical with the rock described by Paul is clearly indicated by his detailed description and the photomicrograph which accompanies his paper. Unfortunately my own determinations are not in agreement with those of Paul, and the purpose of the present note isto state the nature of these discrepancies. The rock is a felspar-free nepheline-basalt of which the chief constituents are idiomorphic nepheline, a titaniferous augite, olivine (chrysolite), the unknown mineral of Paul, biotite, apatite, magnetite, perofskite, and minor amounts of interstitial analcime. The described eudialite has not been discovered, and from the description attached to the photomicrograph this mineral appears to have been confused with nepheline. I find further that the properties of the unknown mineral agree, not with those of any of the artificial calcium orthosilicates, but with those of monticellite. The two olivines are readily distinguished in thin section, the chrysolite occurring as chadacrysts and the monticellite as oikocrysts, the latter appearing as shapeless grains often surrounding chrysolite and enclosing numerous idiomorphic crystals of nepheline and magnetite, and thus belonging to a late stage in the crystallization history of the rock. The chrysolite moreover is readily distinguished—apart from these textural differences—by its higher birefringence and larger optic axial angle (near 90°).