DALY. — THE NATURE OF VOLCANIC ACTION'. 103 



ably basaltic and little modified in composition from its original con- 

 dition. The syntectic-differentiation theory is so strong that the writer 

 is disposed to prophesy its ultimate victory in the competition among 

 explanations of the igneous magmas and rocks, 



Since the lava column of every volcanic vent is an offshoot from an 

 abyssal injection, the lava may represent either the primary basalt, or 

 one of its differentiates, or syntectic material, or a differentiate from 

 syntectic material. The rapid chemical variations in the extrusive 

 magma at the average central vent shows that the conditions are here 



N 



ZONE. OF C\NOLR CONES 



3/ope o/^ 



'auno loc? ^,,^-<z:ONE OF A A lava^ 



:ONE OF PAHO^HO^ LA\/A 



FiGfKB 11. Diagrammatic projection of Maana Kea, illiLstrating the field 

 character of the rocks encountered in cUmbing from sea-level to thesununit at 

 4213 meters above sea. Length of section 55 kilometers. Vertical scale twice 

 the natural. 



specially favorable for differentiation. Two of these conditions are 

 implied in the essential meclianism of central eruption. The upward 

 passage of juvenile and resurgent gases in great relative abundance, 

 lowers the " point " of .solidification of the magma, increases the fluidity, 

 and probably in still other ways aids in magmatic splitting. Secondly, 

 the alternation of active and dormant periods means that the top of the 

 lava column passes repeatedly through the narroAv range of tempera- 

 ture Cjust above the crystallization point), where differentiation is most 

 likely to take place.** High superheat is opposed to magmatic splitting. 



Each of the.se conditions affects only a small volume of magma at 

 any one time ; if lava representing either pole of the differentiation i.s 

 alone extruded, the volume of that flow must be relatively small. New 

 magma rises in the vent. It may be mixed with that representing the 

 other pole of the differentiation just accomplished. The mixture may 

 be extruded, or it may itself be differentiated before the next outflow. 

 Through absorption of foreign rock the new lava may have a composi- 

 tion unlike that originally differentiated. 



It seems inevitable, therefore, that, at the restless volcanic vent, the 

 ever-changing conditions must make a cone which is chemiciilly heter- 



*' Int rusivc masses normally pass through this temperature range only once 

 before dolidificatiun. 



