166 Notices of Memoirs — Notes on Rochall Island and Bank. 



Under these circumstances it is proposed to give to the material of 

 Eockall a distinctive name and to call it ' Eockallite.' 



Eockallite is defined as a rock having the structure of the 

 ' granite-porphyries ' but belonging to the extreme variety of 

 the soda-bearing acid rocks. It is an segirine-quartz-albite rock, 

 intermediate in structure and composition between the soda-granites 

 on the one hand and the grorudites on the other, and presenting points 

 of analogy with some soda-ceratophyres. Among lavas or effusive 

 rocks, rockallite is probably represented by some quartz-pantellerites. 



With respect to the age and relations of the rock forming the 

 singular isolated mass of Eockall, the materials for arriving at 

 a judgment must be admitted to be very small. If the rock, on which 

 the igneous mass lies and over which it appears to have been thrust, 

 could be examined, and especially if this were shown to be a stratified 

 rock with fossils, the evidence thus afforded would be of great value. 

 At present, however, we have nothing but general analogies to guide 

 us. The nearest resemblance to rockallite is found, as we have seen, 

 among the Post-Silurian dykes in the Christiania district so admirably 

 investigated by Professor Brogger ; and it may be that in Eockall 

 we have a similar intrusive mass of the same period that has 

 escaped destruction by denudation. But it must be remembered 

 that among the Tertiary rocks of the Western Isles of Scotland, in 

 St. Kilda (which is the nearest land to Eockall), in Skye, Ardna- 

 murchan, Mull, and Arran, we have granites and granite-porphyries 

 containing soda-augites and soda-amphiboles. Some rocks, indeed, 

 approaching in character to rockallite, occur among the Tertiary 

 eruptives, and it is, therefore, by no means impossible that rockallite 

 may belong to the same recent period. 



In another division of the memoir before ns, Professor Grenville 

 A. J. Cole describes the rock- specimens dredged on the Eockall 

 Bank. These all show a remarkable uniformity of character, as if 

 the original source of the material had been true local rock-masses 

 rather than any ice-drift. 



The great mass of the specimens appears to be basalt of the 

 ordinary type. Some of these appear to be destitute of olivine, 

 and approximate to augite andesite or basalt andesite. Some are 

 vesicular ; others are more or less glassy in texture, and exhibit 

 flow-structures. An andesitic pumice and a scoriaceous andesite, 

 " well removed from the extreme basaltic types," were also found. 

 With these igneous rocks was found one example of a fine-grained 

 grey argillaceous and slightly micaceous sandstone, strikingly 

 resembling the Ordovician sandstones in the altered strata of 

 County Down. 



Among specimens supplied by Mr. Cordeaux of Grimsby, and 

 obtained by fishermen entangled in their lines when fishing on the 

 Eockall Bank, are many basalts ; also one specimen of an ordinary 

 red granite with micro-pegmatite and a red sandstone, very similar 

 to the Torridonian rocks. 



Mr. E. H. Scott supplies to the memoir an account, illustrated by 

 sketch charts, of the winds and currents in the district in which 



