DALY. — THE NATURE OF VOLCANIC ACTION. 



99 



than 450 meters in diameter. The size of the lava conduit is often 

 not shown by the maps of many volcanic "necks." For example, the 

 largest Scottish " necks " described by Geikie are chiefly composed of 

 pyroclastic materials and may represent erosion sections through explo- 

 sion funnels with their characteristic flare. Without even allowing for 



PLAN OF NECK 



Figure 10. Section and ground plan of b;isalt-lava neck in a lateral gorge of 

 the lao valley in West Maui, illustrating the cylindrical form due to gas- 

 fluxing. This w;us one of th(,' subsidiary vents on the flanks of the great We^t 

 Maui cone. There is no trace of faulting in the ash-and-flow series and it is 

 possible that this neck represents the local enlargement (by gas-flu\ing) of 

 one of the dike fi-ssures now visible in the canyon. Nearly natural scale; 

 major diameter of the neck about 50 meters. 



that uncertainty, the writer believes that the conduits of central erup- 

 tions, measured below the explosion zone, have an average diameter 

 of much less than one kilometer. The term "conduit" here means, 

 of course, the pipe above the primary abyssal injection. The main 

 magma chamber may be indefinitely greater in horizontal section. 



We may conchide that the conduits of central eruptions are always 

 small and of the order of magnitude appropriate to the gas-tluxuig 



