GEOPHYSICAL LABORATORY. 147 



flow. The crystallization, which begins soon after extrusion, is soon brought 

 to an end by the rapidly increasing viscosity, so that much, or the greater 

 part, of the lava solidifies as glass. As there are few crystals to serve as 

 nuclei for its escape, the magmatic gas still present is driven out of solution 

 by the increasing viscosity very uniformly throughout the mass and, the 

 greater part being unable to escape, forms the numerous small, spherical 

 bubbles. 



Aa lava, on the other hand, issues at a probably lower temperature than 

 pahoehoe and contains much magmatic gas in solution. This dissolved 

 gas renders the lava very fluid, so that crystallization is easy and rapid. 

 The crystallization maintains the gas pressure and gas content of the con- 

 tinuously diminishing, still liquid portion, which thus remains constantly 

 very fluid, so that crystallization is continuous up to, or nearly up to, the 

 point of complete solidification. The fluidity of the liquid portion is further 

 maintained, and crystallization is promoted, by the fact that, in basalts, 

 the feldspars tend to crystallize earlier than the pyroxenes, so that the liquid 

 becomes progressively more femic and therefore more fluid, thus tending to 

 compensate for diminishing temperature. The latent heat given out by the 

 crystallization also tends to maintain the temperature. As there are many 

 crystals to act as collecting nuclei for the magmatic gas that is driven out of 

 solution, large bubbles are formed by coalescence, which rapidly rise through 

 the fluid mass, producing the abundant clouds of steam and gas that accom- 

 pany aa flows and also freeing from bubbles the lower portions of the flow. 

 Because of the greater fluidity of the liquid portion, aa lava is, in general, 

 able to flow for a longer distance and more rapidly than pahoehoe. 



(511) Die ICrystallstruktur des metallischen Iridiums. Ralph W. G. Wyckoff. Z. Ivryst., 

 59, 55-61. 1923. 



The face-centered cubic arrangement previously assigned to metallic 

 iridium is confirmed and a more accurate method is outlined for determining 

 the fundamental dimensions of a crystal by powder photography. This 

 procedure involves a mixing of sodium chloride, or other standard comparison 

 material, directly with the substance under investigation. Its application 

 to iridium of the highest purity yields a length of side of the unit cube: 

 a = 3.823 A. u. 



