Geology of Rockall. 157 



last vestiges of the famous vanished continent. Modern research has, how- 

 ever, quite disposed of this theory. 



' Nevertheless, Rockall is of considerable interest, especially to geologists. 

 In 1810 Basil Hall, then a young officer in H.M.S. Endymion, obtained a 

 fragment from this rock, which later found its way into the collection 

 of the Geological Society. More than 30 years afterwards, the specimen 

 was recognized; it was then mislaid for another 30 years, and in 1895 was 

 brought to me by the late Prof. T. Rupert Jones. 



' He not only carefully 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 by two of the 

 officers of H.M.S. Porcupine in 1868 during the survey of the North Atlantic. 

 The microscopic study of these specimens shows that in Rockall there exist 

 rocks of exceptional interest, which are not represented in our islands, but 

 have analogues in the Christiania district of Norway, where they have 

 been so well studied by Prof. W. C. Brogger. These rocks, as shown by 

 microscopic study and by a chemical analysis made by Mr. Makins, consist 

 essentially of three minerals — quartz, the felspar albite, and the rare soda- 

 pyroxene aegirite, with its dimorphous form acmite. The rock, therefore, re- 

 sembles the soda-granite and the grorudite of Prof. Brogger — but, in deference 

 to the opinion of the distinguished Norwegian petrographer, a distinct name 

 was given to it. 



• In 1896 an attempt was made to obtain further specimens of the rocks of 

 this islet by members of the Royal Irish Academy ; but, although many 

 valuable observations were recorded, it was found, after two voyages had 

 been made to Rockall, quite impossible to land and obtain specimens. 



' Dredging operations have yielded many specimens from the Rockall Bank, 

 and these were examined by the late David Forbes and Prof. Grenville A. J. 

 Cole. The abundance of basalt-fragments among these dredgings suggests 

 the possibility of Rockall belonging to the same petrographical province as 

 St. Kilda, Iceland, the Inner Hebrides, and the North of Ireland ; hitherto, 

 I believe, no rocks resembling "rockallite" have been found in this province. 

 On the other hand, 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, which includes the Christiania district and 

 the Scottish Highlands. 



' Some months ago Prof. Iddings and Dr. Washington represented to me 

 the desirability of a more detailed analysis of this rock. One of the two 

 small fragments available was, by the advice of Prof. Watts, sent to America 

 by the Council of the Imperial College of Science & Technology, to whom 

 they belonged, and the paper now about to be read gives the result of the 

 study of this minute specimen by Dr. Washington.' 



The following communication was read : — 



' The Composition of Rockallite. 1 By Dr. Henry S. Washington, 

 For.Corresp.G.S. 



The paper centres around a detailed chemical analysis made bv 

 the author of the type-material of rockallite, an igneous rock of 

 exceptional chemical and mineralogical composition. 



A petrographical account is given of the rock, with special 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 aegirite and a pale yellowish-brown acmite. Some zircon is 

 present, but not to more than a tenth of 1 per cent, of the rock. 



