82 PROCEEDINGS OF THE AMERICAN ACADEMY. 



depth at least 300 meters below the original top of this lava column. 

 Ransome (in Bulletin 303 of the United States Geological Survey, 

 1907, p. 68) states that the bottom of a single 320-raeter flow of basalt 

 at Eldorado Canyon, Nevada, is vesicular. These and other known 

 examples seem to strengthen the belief that bubbles may form in 

 magma at the depth of several kilometers. 



It is important to note that two-phase convection has two distinct, 

 though related causes. Principal stress has hitherto been laid on dif- 

 ferential vesiculation in depth, whereby a mass of magma becomes more 

 buoyant than the enclosing magma and rises. Just as inevitably, the 

 magma which is freed of gas at the crater, must sink and stir the 

 column to great depth. Even if the column is not vesiculated at all, 

 this second mode of convection is likely to be effective in the vertical 

 transfer of the magma. As a rule, the density of a liquid is lowered by 

 the absorption of hydrogen, nitrogen, oxygen, or other relatively light 

 gas. This is very probably true of natural mixtures of juvenile gases 

 when dissolved in magma. As these gases stream or diffuse from all 

 azimuths in the feeding chamber toward the base of the narrow con- 

 duit, they are there concentrated. Thus, the magma in the conduit, at 

 its lower levels, attains a density less than that of the average magma 

 of the feeding chamber, and, a fortiori, less than that of the gas-freed 

 magma descending from the crater level. This is another kind of den- 

 sity convection depending on the relative concentration of juvenile gas. 

 For lack of experimental data, it is now impossible to estimate the 

 efficiency of this species of convection. It may be a powerful ally of 

 two-phase convection proper. For example, it is conceivable that the 

 upward movement of magma is begun in the conduit because of the 

 concentration of gas in solution and not in bubble phase. Then, as 

 the magma rises to levels of smaller pressure, the gas begins to separate 

 out in bubbles and enforces true two-phase convection of ever-increas- 

 ing speed. In view of these various modes of gas-control, the vertical 

 stirring of the magma column may, perhaps, be more safely described 

 as, in general, a gas-concentration convection. Yet, the actually ob- 

 served fact is that, at the crater, the gaseous phase does separate, in 

 bubble form, from the liquid phase, and the writer has preferred 

 to emphasize this empirical fact in adopting the name "two-phase 

 convection." 



Lava Fountains. — Herein the writer believes that we have an essen- 

 tial part of the explanation of " Old Faithful," the site of the greater 

 periodic "fountains " of Kilauea. (Plate II. B.) That circular area, 

 about twenty meters in 1909, has represented the true axis of the lava 

 column for many years, and seems to have been the main source of 



