1885.] Agency of Water in Volcanic Eruptions. 259 



i 



long, would allow the lava to consolidate, leads the author to believe 

 that the outer solid crust may be less even than 20 miles thick. 



That the crust does possess great mobility is shown by the fact 

 that since the glacial period, there have been movements of conti- 

 nental upheaval — to at least the extent of 1,500 to 1,800 feet — that 

 within more recent times they have extended to the height of 300 to 

 400 feet or more, and they have not yet entirely ceased. 



With regard to the suggestion of the late Professor Hopkins that 

 the lava lies in molten lakes at various depths beneath the surface, 

 the author finds it difficult to conceive their isolation as separate 

 and independent local igneous centres, in presence of the large areas 

 occupied by modern and by recently extinct volcanoes. But the 

 chief objection is, that if such lakes existed they would tend to 

 depletion, and as they could not be replenished from surrounding 

 areas, the surface above would cave in and become depressed, 

 whereas areas of volcanic activity are usually areas of elevation, and 

 the great basaltic out-wellings of Colorado and Utah, instead of being 

 accompanied by depression, form tracts raised 5,000 to 12,000 feet 

 above the sea-level. 



These slow secular upheavals and depressions, this domed elevation 

 of great volcanic areas, the author thinks most compatible with the 

 movement of a thin crust on a slowly yielding viscid body or layer, 

 also of no great thickness, and wrapping round a solid nucleus. The 

 viscid magma is thus compressed between the two solids, and while 

 yielding in places to compression, it, as a consequence of its narrow 

 limits, expands in like proportion in conterminous areas. As an 

 example, he instances the imposing slow movements of elevation 

 which have so long been going on along almost all the land bordering 

 the shores of the Polar Seas, and to the areas of depression which so 

 often further south subtend the upheaved districts. 



With respect to the primary cause of these changes and of the 

 extravasation of lava, the author sees no hypothesis which meets all 

 the conditions of the case so well as the old hypothesis of secular 

 refrigeration and contraction of a heated globe with a solid crust, — 

 not as originally held, with a fluid nucleus, but with the modifications 

 which he has named, and with a quasi rigidity compatible with the 

 conclusions of the eminent physicists who have investigated this part 

 of the problem. Although the loss of terrestrial heat by radiation is 

 now exceedingly small, so also is the contraction needed for the 

 quantity of lava ejected. Cordier long since calculated that sup- 

 posing five volcanic eruptions to take place annually, it would require 

 a century to shorten the radius of the earth to the extent of 1 mm. 

 or about -~ inch. 



The author, therefore, concludes that while the extravasation of the 

 lava is due to the latter cause, the presence of vapour is due alone to 



