59° 



CLARENCE N. FENNER 



The banding present in the lava of the dome that now rises 

 above the vent gives very direct evidence regarding the mechanism 

 of its extrusion. This banding is visible at short intervals around 

 the outer circumference of the dome, where masses of rock in place 

 protrude through the general heap of disrupted blocks, and its 

 direction is found to be parallel with the circular outline of the 

 dome. There is also very good evidence of a process of exfoHation 

 of the outer layers as the central core was forced upward.' The 

 character of fracture surfaces of the lava-blocks of the dome is 



Fig. 14. — "Cornice structure" in Novarupta lava, produced by fracturing and 

 viscous yielding of the hot mass. Photograph by R. F. Griggs, 1919. 



interesting. It shows that many of the fractures occurred while 

 the glass possessed properties of both brittleness and viscosity, 

 such as are shown by stiff tar. This resulted in effects of the kind 

 shown in Figure 14, where the surfaces formed by intersecting 

 fractures have become wrinkled and fluted. The term "cornice 

 structure" suggested itself at once as appropriate for such features. 

 Many bread-crust bombs are found in the vicinity of Nova- 

 rupta. These were ejected from the crater as masses of non- 

 vesicular, plastic glass, and the vesicularity developed during the 



' Compare Harker, The Natural History of Igneous Rocks (1909), p. 58, Fig. 8. 



