INTERESTING PHENOMENA ON THE ERUPTION OF USU 287 



sion to be a laccolith. Many geologists agree with the theory of 

 G. K. Gilbert as to the formation of the laccolith. It is a plausible 

 supposition that the propelling magma would find the line of 

 least resistance in certain planes, lifting the land above it. In the 

 early stages of the activity in Mount Usu enormous quantities of 

 gases were emitted, together with ashes and bombs, while at the 

 mature stage the north side of the second zone of craters was 

 sharply elevated in a straight line. Ernest Howe 1 made experi- 

 ments on the intrusion 

 of wax into plaster, 

 marble, sand, and coal 

 layers, and demon- 

 strated how the lacco- 

 liths are formed. Where 

 there are fissures from 

 the inner source to the 

 surface, there must be a 

 line of least resistance 

 at this place. Intrusion 

 between the strata 

 occurs only where there 

 are no fissures or cracks extending to the surface. It is unreason- 

 able to believe in the intrusion of the laccolith while the distinct 

 cracks shown in the two zones of craterlets are in evidence, as we 

 have seen in the experimental data of Howe. The majority of 

 nearly 150 laccoliths in the western part of the United States of 

 America are composed rather of acidic rocks, while rock 2 as basic 

 as that of Usu is found only in rare cases. The basic lavas pre- 

 serve a comparatively high degree of liquidity down to rather low 

 temperatures, with a quick process of solidification by rapid crys- 

 tallization, as is well illustrated in blast-furnace slag. Ejection of 

 many bombs demonstrated that the lava was not seated in the 

 great depths. 



From the facts stated above, the writer is inclined to believe that 

 the formation of a "plug," elongated west-northwest by east- 



1 Twenty-first Ann. Rept., U.S.G.S., Part III (1901). 



2 See the analysis of bomb. 



Fig. 27. — A diagram showing the intrusion of 

 "plug": A, B, C, D, mountain slope before the 

 eruption; B, E, F, elevated mountain; C, shore 

 line before the eruption; F, present shore line. 



