294 DE. H. S. WASHINGTON ON THE [June I914, 



11. The Composition of Eockallite. By Henry Stephens 

 Washington, Ph.D., For. Corr. Greol. Soc. (Read March 

 25th, 1914.) 



Introduction. 



An igneous rock of quite exceptional chemical and mineral com- 

 position and of unusual petrographic interest is the granitic 

 rockallite. This forms, in part, the small islet of Rockall, situated 

 in lat. 57° 36' 1ST. and long. 13° 42' W., about 170 miles west of the 

 Hebrides, and about half-way between Ireland and Iceland. The 

 haycock-shaped islet is almost inaccessible, and only three specimens 

 of the rock are known to be in existence. To quote from a letter 

 (dated December 9th, 1913) received from Prof. J. W. Judd, 

 C.B., F.R.S. : 



' Although a number of attempts have been made to examine Rockall, yet, so 

 far as I know, only two landings upon it have been effected. Even in these 

 two cases it was only possible for sailors to jump from a boat on to a pro- 

 jecting point of the rock, and to knock off pieces which they threw into the 

 boat. A carefully-equipped expedition made two voyages to the rock in 1896, 

 but on both occasions had to give up all attempts at landing upon it.' 



The rock has been described petrographically in great detail by 

 Prof. Judd, 1 who examined all three specimens, in a paper which 

 forms one of the reports of the Irish expedition of 1896. To these 

 reports the reader may be referred for descriptions of the islet and 

 other information. 



Prof. Judd's paper contains a chemical analysis of the ' Inskip ' 

 specimen, made by Mr. C. J. S. Makins. As this analysis is far 

 from complete, ferrous oxide, titanium oxide, and water not having 

 been determined, and only a ' trace ' of potash reported, and as the 

 rock is of so entirely exceptional a character, it was thought that 

 a new and complete chemical analysis was desirable. Through the 

 kind offices of Prof. Judd and the courtesy of the Governors of 

 the Imperial College of Science & Technolog} r , about half of the 

 Inskip specimen (that analysed by Makins) was given to me for 

 examination and analysis. It is not only a duty but a pleasure for 

 me to acknowledge here my deep sense of gratitude to, and appre- 

 ciation of the great courtesy and liberality of, these gentlemen for 

 the opportunity to study this exceptionally valuable material. I 

 can but feel gratified that the results of my examination not only 

 confirm the observations of Prof. Judd and Mr. Makins, but have 

 also added some new and rather important features of interest to 

 the rock. 



The specimen, as received from Prof. Judd, weighed about 

 17 grams. Two thin sections were cut, and a fragment of about 

 6 grains (with one thin section) has been deposited in the Petro- 

 graphic Reference Collection of the United States Geological 



1 Trans. Eoy. Irish Acad. vol. xxxi (1897) pp. 48-58. See also Geol. Mag. 

 dec. 4, vol. vi (1899) pp. 163-67. 



