238 /. H. L. VOGT 



cases, may be measured approximately by the viscosity valid to 

 the dry melts. 



According to this we find that basalt magmas commonly have 

 flowed out in quite thin streams, which may have covered very large 

 areas, even several tens of thousands of square kilometers, while 

 acid rocks, especially rhyolites and related acid effusives, often, 

 but not always, form domes. 



We will here also mention another point of great geological impor- 

 tance. As maintained by R. A. Daly in his Igneous Rocks and 

 Their Origin (19 14), among the deep-seated rocks the acid rocks 

 (granite and granodiorites) have an extension several times as 

 large as the basic rocks (with the gabbros as the most important 

 representative). The contrary is the case with the effusives since 

 here basaltic (and pyroxene-andesitic) rocks quantitatively are 

 much more important than rhyolites. I consider from my own 

 geological experience, that Daly is right in this view, but I do not 

 agree with Daly's explanation. 



We must suppose that the magmas of the granitic deep-seated 

 rocks generally contained a much larger quantity of the Hght 

 volatile compounds than the magmas of the gabbroic rocks, and 

 consequently that the temperature at the beginning of crystalliza- 

 tion compared with that of the anhydrous melts was decreased 

 much more for the granites than for the gabbros.^ 



When an acid (or granitic) magma, at the temperature of the 

 upper limit of the crystallization interval (see the next chapter), 

 comes under a lower pressure, by the passage to the surface of the 

 earth, a great part of the light volatile compounds will escape. 

 By this the temperature of crystallization increases and at the 

 same time also the viscosity increases; the latter even to a consider- 

 able degree. The magma, in the first stages of crystalKzation 

 will under these circumstances become so little movable or so viscous 

 that part of it will never reach quite to the surface, and part of it 

 will do so only under favorable conditions. For the basic (gabbroic 

 or basaltic-andesitic) magma we must also assume some escape of 

 the light volatile compounds by the passage to the surface. This 

 is of less importance, however, since the volatile components here 



^ See their Journal for 1922, pp. 664-71. 



