DALY. — COMPOSITIONS OF IGNEOUS-ROCK TYPES. 239 



together those of plutonics and the corresponding effusive rocks. The 

 comparisons show the truth of Rosenbusch's statement that the 

 effusives are, on the whole, somewhat higher in silica and alkalies 

 and lower in iron oxides, lime, magnesia, etc., than the respective 

 plutonics. 



The importance of this rule is at least two-fold. It proves the 

 value of Rosenbusch's primary division into the deep-seated types and 

 the surface lavas. It shows therewith one of the reasons why the 

 Norm 6 Classification of igneous rocks is largely a failure so far as either 

 the field-geologist or the student of petrogeny is concerned. 



Secondly, the rule suggests clearly that at volcanic vents there is a 

 general cause for the removal of iron, magnesium, and calcium oxides 

 from the magmatic columns and that the cause is more effective in vol- 

 canic vents than in the average plutonic body. The cause is most 

 probably to be found in the gravitative settlement of part of the ferro- 

 magnesian and other constituents of early crystallization. These con- 

 stituents may settle out either as solid crystals or as liquid fractions 

 immiscible near the consolidation point of the magma. Since, on the 

 average, the column of fluid magma is taller in an active volcanic 

 vent than in a plutonic mass, the overlying phase of the splitting 

 magma should be, in general, slightly more acid and alkaline than the 

 corresponding pole of differentiation in a deep-seated mass. In the 

 nature of the case the more acid-alkaline pole is the one most liable to 

 flow out at the surface. Though volcanic vents are much narrower 

 than plutonic chambers and therefore subject to quicker chilling, with 

 a resulting check to differentiation, this tendency is largely counter- 

 balanced by the passage of very hot gases through vents. The mere 

 agitation in the vents facilitates the separation. Whatever additional 

 considerations are necessary to complete the comparison, it must here 

 suffice to note that, as a rule, the laws of solution as applied to 

 magmas seem to demand a differentiation with slow cooling, whereby 

 a surface lava is less basic and ferromagnesian than the plutonic body 

 feeding the vent of that lava. The corroboration of Rosenbusch's 

 above-mentioned rule through the world-averages appears, therefore, to 

 be of use in illustrating one of the world-wide influences controlling 

 the origin of igneous rocks. 



Some special conclusions regarding classification may be noted. 

 From the averages it is evident that dacite is the effusive correspond- 

 ent of granodiorite and not of quartz diorite. The contention of 



6 Quantitative Classification of Igneous Rocks, by W. Cross, J. P. Idd- 

 ings. L. V. Pirsson, and H. S. Washington, Chicago and London, 1903. 



