354 Henry Woodward — On Volcanos. 



the notion of a globe witli a moderately thick crust, and a fluid 

 interior. 



In order to produce a complete accordance in the motion of the 

 entire mass, it is necessary, according to these authorities, to assign 

 a solid crust of at least 800 to 1000 English miles in thickness. 

 Can any one believe that lava is pressed up through channels of that 

 length ? 



M. Delaunay has, however, clearly shown that " there is a funda- 

 mental error, not in the mathematical formula, but in the condition 

 assumed, namely, that the interior is filled with a free-flowing liquid 

 rock. The interior fluid can only be of the nature of lava, and that, 

 when examined at the surface, however fresh, is a very intractable 

 mass, flowing indeed as does thick honey, pitch, or slag ; incapable 

 of moving, at the very utmost, above a few miles an hour, even on 

 a slope of 30°, and on ordinary slopes only one mile, half a mile, or 

 even thirty or forty feet in an hour. In this condition it would 

 obey perfectly the motions of the solid crust. The problem solved 

 by Mr. Hopkins, looked at in this light, does really not settle any- 

 thing as to the thickness of the earth's crust." (Phillips, " Vesuvius," 

 p. 332.) 



"The globe is continually, though very slowly, losing heat; it 

 grows cooler in a very small degree, and sufiers contraction in the 

 same small degree. From what we know certainly of the constitu- 

 tion of the crust of the globe it is of unequal strength to resist 

 change of form in different parts. The weakest part must yield, 

 and if by local yielding the general pressure may be satisfied (which 

 is equivalent to supposing the general pressure determined to a small 

 area), the displacement of small tracts may be extremely great, and 

 the rocks be bent into arches and broken by faults." 



" If we are right in our views of the history of the globe, very 

 many epochs would arise, where, first in one region, then in another, 

 lines or areas of relative weakness would be depressed into concave 

 seas, and receive a long series of deposits ; and at other times the 

 same areas, or parts of them, might be re-elevated, producing end- 

 pressures and violent local flexures or fractures, resulting in earth- 

 quake shocks and volcanic eruptions." (Ibid., p. 335.) 



Viewed by the light which volcanic action affords us in other parts 

 of our globe at the present day, the geologist sees the simplest 

 explanation possible by which to understand the upheaval of the 

 sedimentary rocks. 



But it is not where volcanic force exhibits itself at the surface, 

 escaping in jets of steam and vapour, or even in lava-flows, that we 

 look for the greatest proof of work accomplished. As well might 

 we study the smoke-stack of a steam-engine to judge of its horse- 

 power. No ; it is by the consideration of the elevatory force exerted 

 to raise such vast masses as the Himalayas, or the Peruvian Andes, 

 that we can best appreciate the work achieved by upheaval. 



Nay more. Not an inch of land could remain permanently above 

 the sea-level but for its silent yet mighty support. And when we 

 look back through Mesozoic to Paleozoic times we see its active and 



