Abstract
Geological Society, March 25.—Dr. A. Smith Woodward, president, in the chair.—Prof. J. W. Judd: The geology of Rockall. Rockall is a small isolated rock in mid-Atlantic, lying 184 miles west of St. Kilda, and, except in the calmest weather, is inaccessible. The rock rises from a bank (the “Rockall Bank”) upon which there are dangerous reefs. In 1810 Basil Hall, obtained a fragment from this rock, which later found its way into the collection of the Geological Society. More than thirty years afterwards, the specimen was recognised; it was then mislaid for another thirty years, and in 1895 was brought to the author by the late Prof. T. Rupert Jones. He not only studied all the literature connected with Rockall, but was able to trace two other specimens of the rock, the loan of which he obtained and brought to me. They had been procured in 1868 during the survey of the North Atlantic. The microscopic study of these specimens shows that in Rockall there exist rocks of interest, not represented in our islands, but which have analogues in the Christiania district of Norway. These rocks consist essentially of three minerals—quartz, the felspar albite, and the rare soda-pyroxene ægirite, with its dimorphous form acmite. Dredging operations have yielded specimens from the Rockall Bank. The abundance of basalt-fragments among the dredgings suggests the possibility of Rockall belonging to the same petrographical province as St. Kilda, Iceland, the Inner Hebrides, and the north of Ireland. The existence of borolanite and other alkaline rocks in the northern Highlands suggests the possibility of Rockall being the western extension of a much older province. Some months ago Prof. Iddings and Dr. Washington represented to the author the desirability of a detailed analysis of this rock. One of the two fragments available was sent to America, and the following paper gives the result of its study by Dr. Washington.—Dr. Henry S. Washington: The composition of Rbckallite. A petrographical account is given, with reference to the influence of the constituent minerals upon the bulk-analysis. Rockallite has a fine-grained granitic structure, and is composed of about equal amounts of colourless quartz, alkaline felspar, and soda-pyroxene. The pyroxene is of two kinds: a bright grass-green ægirite and a pale yellowish-brown acmite. Some zircon is present. A chemical analysis has been made, zirconia and the rare earths being especially looked for. Several new points of interest have presented themselves. The outstanding features appear to be the high percentages of silica, ferric oxide, and soda, and the low percentages of alumina, ferrous oxide, magnesia, lime, and potash. The interest of the new analysis, however, lies in the detection of zirconia and cerium oxide in large amounts: the percentage of cerium oxide being larger than that from any known igneous rock, with the exception of the nepheline-syenite from Almunge in Sweden. The norm has been calculated from the old and the new analyses, and the author finds that the rock falls into the subrang rockallose with the general symbol III. 3. 1. 5. These analyses are the only representatives of the subrang rockallose among the 8000 analyses of igneous rocks that the author has now collected. It is proved that the zirconia and cerium oxide enter into the composition of the pyroxenes.
Article PDF
Rights and permissions
About this article
Cite this article
Societies and Academies . Nature 93, 154–155 (1914). https://doi.org/10.1038/093154a0
Issue Date:
DOI: https://doi.org/10.1038/093154a0