154 SCIENCE PROGRESS. 



abscissae and those of the various bases as ordinates for 

 their respective diagrams. 



Attempts have been made to verify directly the change 

 in the material erupted by a single volcano through long 

 periods. Lang 2 has critically examined the cases of 

 Vesuvius and Etna, for both of which we have a large 

 number of chemical analyses, even when the older ones, 

 conducted with imperfect analytical methods, are eliminated. 

 Fuchs had already concluded (1870) that, in spite of the 

 complex and peculiar mineralogical constitution of the 

 Vesuvian lavas, the chemical composition of the different 

 flows erupted in historic times is almost exactly the same ; 

 and von Lasaulx (1880) had arrived at a very similar 

 conclusion for the Etna lavas. Lang, however, makes a 

 more searching examination of the analyses, laying special 

 stress on the relative proportions of lime, soda, and potash 

 given by the bulk-analyses of the several rocks. From a 

 discussion of fifty-five trustworthy analyses of different lavas 

 of Vesuvius, he finds that their mean composition for 

 different periods of fifty years does vary sensibly, and, in 

 particular, that the lavas of 1740 to 1790 were richer in 

 alkalies and silica, and poorer in lime than those which 

 preceded and followed. This variation, however, is not 

 great, and is evidently not progressive. He also notes 

 during each period a slow variation which does appear to 

 be progressive : thus the lavas of the last hundred years 

 show on the whole a gradual increase in the proportion of 

 lime and a corresponding falling off in potash. Again, in 

 the Etna lavas from 1766 to 1879, the author points out a 

 gradual increase in lime as compared with the alkalies, the 

 ratio rising progressively from 1*36 to 2*88. Despite the 

 small variations noted, the general impression obtained from 

 Lang's tables is that of remarkable constancy in the com- 

 position of the lavas from one vent through many centuries : 

 for instance, the silica-percentage of the rocks poured out 

 from Vesuvius has scarcely changed since the days of 

 Pompeii. The conclusion is that the processes which 

 produce changes act very slowly. By taking thousands 

 instead of hundreds of years, we might expect to find greater 



