222 A. ffarker — British Igneous Rocks. 



figure is very distinctly higher than Clarke's. We see next that 

 the British rocks are poorer in magnesia, and rather poorer in soda 

 than the American, while in lime and potash the two columns show 

 no great disparity. The alkalies jointly amount to 599 in the 

 one column as compared with 644 in the other. We might have 

 anticipated a greater deficiency in alkalies in the British rocks, and 

 also a more marked excess of lime, as compared with the American, 

 since the majority of our rocks belong decidedly to what Iddings has 

 styled the 'Sub- Alkali ' rather than to the ' Alkali ' group, while both 

 groups are well represented in the United States. The apparently 

 rather large percentage of water in column II must be due in part 

 to the circumstances already noted. In phosphoric acid there is a 

 sufficiently close agreement between the two averages. 



If we were to separate the English, Scottish, and Irish rocks, and 

 calculate averages for the several groups, we should find that these 

 would be disturbed by principles of selection which have entered 

 into the choice of the rocks analyzed. Thus we have a large 

 number of analyses of Irish granites by Professor Haughton, while 

 the Scottish granites have been comparatively neglected. This goes 

 to make the Irish average more acid and the Scottish more basic 

 than the general mean. Nevertheless, it would no doubt be possible 

 to get results of interest for even smaller areas, provided that rocks 

 sufficiently various in kind are included ; and, if we take account of 

 partial as well as complete analyses, we shall find for some areas a 

 considerable body of data. For example, 96 igneous rocks from the 

 English Lake District give a mean silica-percentage 58-75, not very 

 different from that of the general average ; but 82 examples from 

 the Malvern Hills give only 54-81. The low figure in the latter 

 case can scarcely be accidental. It is worthy of note, too, that of 

 78 igneous rocks from the Malverns, Timins estimated copper oxide 

 in 21 and recorded traces in 11 others, the mean percentage being 

 0-23 for these 32 rocks, or 0-12 for the whole 78. This is at least 

 twenty times as great as any probable estimate for the general 

 average of rocks. 



The mean specific gravity of a large number of igneous rocks 

 from a given district will probably be found in some eases to be 

 characteristic. Of the 397 analysed British rocks included above, 

 specific gravities are given for 209, the mean value being 2-777. 

 By including 527 miscellaneous igneous rocks from various districts 

 of Britain, we find the mean specific gravity of the whole 736 to 

 be 2-763. Taking the two special districts already mentioned, we 

 find that 113 Lake District rocks give a mean specific gravity 2-737, 

 decidedly below the average, while 38 from the Malverns give 2-841, 

 which is decidedly above the average. The preponderance of basic 

 rocks in Skye comes out less strikingly, 332 miscellaneous igneous 

 rocks from that island giving a mean of 2774. In all such 

 calculations it is of course essential that no selection should be 

 exercised, the rocks being taken at random so as fairly to represent 

 the variety found within the district. 



